Bitcoin is not open source because only a relatively small cross section of Bitcoin users understand the programming.

I use Bitcoin Core for Windows found through the link at https://bitcoin.org/en/download. The name of the current version file that I download is "bitcoin-0.13.1-win64-setup.exe." The download screen says that the size of the file is 12.5 MB (megabytes). The Windows (zip) of the same program says it is 22.6 MB. I use the smaller of these two.

While I really don't know what the difference is between these downloads, the smaller of the two installed to my "Program Files" directory in my computer, at 41 MB. (The whole blockchain is well over 100 GB (gigabytes).)

Do you know how many lines of code you could write to fill 41 MB of computer hard drive memory? Thousands!

What do all these lines of code do? What do they do to your computer? How do they work with your Internet connection, and what is really being sent over the Internet when you have Bitcoin running? Is there really anybody who knows the whole thing?

The point is, Bitcoin is not really open source, because it is not open to the vast majority of minds that use it. The average person could understand the whole Encyclopedia Britannica easier than he could understand what goes on with the Bitcoin programming in his computer. And probably fewer than 99% of programmers understand it, to say nothing of lay people.

OP is having issues with:
updated documentation that actually explain it properly
code lacking comments.
signposting to the best source of code, comments, documentation.

though it is open source, the organisation of HOW its open is missing.
back in my day nearly every line of code had comments to explain what it does.
back in my day nearly every line of code had namespaces/variables with WORDS that explained what it does not just characters

there are many modules that do have comments and variables with understandable names. but there are some without.

EG https://github.com/bitcoin/bitcoin/blob/master/src/secp256k1/src/ecdsa_impl.h (grabbed randomly)
if you see a module assign variables 'b1' 'rs' 'rr' 's1' 'u1' 'u2' 'sn' 'pr' without commenting what they do. it helps no one.

yes with a couple re reads and running it through your mind in what used to be called 'pseudocode' you get the gist of it eventually. but its still badly organised

the funnier part is that many core devs get very snobby if forum posts are not wrote in 100% white paper approved level of English grammar, even when knowing forums are just for common/social communication where only 10% of the planet deem English to be their first language. but their own code lacks the basic coding etiquette

Thank you. This is much closer to the explanation that I was looking for.

the dictionary definition of open source is that the code is not locked away in some vault that only available to authorised people.
BUT
the concept and context of bitcoins openness is that anyone should be able to get involved. which can actually be hindered by the ettiquete some devs have. especially if its required that their code should be peer reviewed.

this is how bugs happen. by not clearly laying out the code. people gloss over the code and just think. 'this guy looks like he knows what he is doing lets just let the code fly' and not actually reading line by line and running scenarios.

its already happened a couple times even after being so called 'peer reviewed'

Since you refer to a Windows executable, your download is a precompiled binary which most certainly includes a few libraries not available in Windows by default. Even if you have full understanding of the Bitcoin protocol and the development tools (including a programming language) with which this specific implementation of Bitcoin was built, you will still be using third party as well as operating system libraries and kernel calls (e.g. network stack calls), and you can't possibly know what exactly all these are doing to your computer, especially if you are using Windows...

In this way, your point is essentially moot (whatever you wanted to say)

The part that is NOT moot is the fact that people need to realize this... that they are trusting all kinds of programmers all over the place, especially Microsoft.

The whole money system has slowly grown into an Internet thing, all run by computers and programming that isn't understood by anybody except the tiniest handful of programmers. The open source aspect of Bitcoin is little different, but not much.

How big is Windows 10 on the computer. Gigabytes. And much of it is NOT open source.

Probably a lot of people would be scared to no end if they realized what their computers are doing behind the scenes. The term "open-source" simply quells their fears, even though little open source programming is really understood by anyone, relatively.

[from wikipedia] Open source as a development model promotes universal access via an open-source or free license to a product's design or blueprint, and universal redistribution of that design or blueprint, including subsequent improvements to it by anyone. [end]

So you can see what's there is behind the code itself, this is way it's "open source".
Even [from wikipedia]  The open-source model is a more decentralized model of production, in contrast with more centralized models of development such as those typically used in commercial software companies.[1] Scientists view the open-source model as a case of open collaboration [end]

It's normal not all people could understand "the code" by itself, but this not means is not an open source code.
About malicious programmer there are some proposal that need to be validated before submitted:
https://en.bitcoin.it/wiki/Bitcoin_Improvement_Proposals
Some proposal cannot be adopted: https://en.bitcoin.it/wiki/Prohibited_changes

Probably with these BIP the protocol is safe, and only an hardfork (and another COINS not actual bitcoin) could help malicious programmer.

Yes, but it has been known for decades already

Even in totally open-sourced systems like Linux or FreeBSD you have no other option but to trust other people and their knowledge, since it is simply impossible to first understand and then check all types of software used in a modern computer. In fact, even if you have a full source code tree of some program or utility and understand every line of it (or developed it yourself), you can't be 100% sure that it will do exactly what you think it should (and I'm not talking about bugs)

All that this says is that these programs, although called open source, are not really as open as the public might think that they are.

many do know programming languages. but if the code uses undescribed variables or just randomly selected characters as variables, where no comments describe what the function does.. it is a headache

there is a major difference between quickly reading code to see if it has any "kill commands" and reading it line by line to see what the code does.
it took many people alot of time to read line for line and translate it in their head into 'pseudocode' and then work out what a function does by simulating it. which all can be solved by some basic etiquette. most of the people that actually bother reading it line by line and do simulations are those that then make their own implementations that run better. due to the boysclub of core being close minded to outside criticism.

the issues arise when one coder does stuff that 'works' but does so in a fancy way which could have been done in other ways. though passing a review test, the peers are not simulating it to think of other ways it can be written. they just put their thumbs up and move on. 'trusting' the dev that wrote it.

there have been many times we have seen bitcoin toyed around with later purely because one function passed a review, but later finding out if a function was wrote differently it would work better.

just because 90 spell checkers put their thumbs up, doesnt mean the code is perfect. doesnt mean all 90 spell checkers have run simulations or thought of different strategies. most of they time they quickly browse the code doesnt have 'kill commands' and then "TRUST!!!!" the dev that wrote it knows what he is doing, thus resulting in a fake positive peer review

its why there are all the core fanboys throwing out doomsdays. they dont run real simlations or actually read code, they just read summaries and other peoples opinions and trust.. trust of one person who trusts another who trusts another even when all 3 have not read a line of code. makes things go wrong.

take a notable name in the forum.. Lauda.. he doesnt know C++ yet has been very vocal of his trust of core code and core devs. and people then trust him. all because the code is not laid out.

take the way devs implemented the tx fee.. not only subverting coded methods of 'priority' which would have alleviated the war, but also using 'averages' which dont make the fee's drop reactively when demand is low. but actually keeps fee's up. even when one block is low demand.
EG take a 25 block average.. imagine first 24 are 0.0001 and the 25th is 0.0025 then look at the 'average' after that.. even if demand was near 0 and no one was pushing the fee up.... the "average" itself pushes up


so although the tx fee passes 'peer review' its sloppily coded in a way that is not reactive to low demand. or cares about using code logic to show real priority. all its doing is pushing/maintaining a high price even when demand is low.

it requires pools to avoid using the 'average' and to accept cheap tx's just to break the cycle that the 'average' fee war game is playing.