I am surprised that BADecker is asking this question. I have seen you as a pretty old member in the Politics & Society section. Still, These are valid questions whenever you try to wrap your head around Bitcoin. I don't claim to know it fully but will try to answer what I understand. So,
Where is the blockchain found on the net?
The blockchain is data like everything else on the internet. The difference is that it does not have single centralized servers storing all of that data. Instead, every single "Full Node" keeps a copy of the blockchain whenever you run the software for the first time. This is why it downloads a few hundred GBs of data.
Blocks are made by the consolidation of all the transaction data that is happening every second. It keeps getting stored and keeps increasing in size with every new block.
Is it maintained by the miners? Can anybody help to maintain the blockchain? How does one become a dedicated blockchain "maintainer?"
Miners do not "maintain" the blockchain and the blockchain does not need maintenance or administration like any other centrally stored data would need. Like I said above, every Full node has a full copy of the blockchain. Whenever someone installs a new instance of the software, they too can opt to keep a whole copy of the blockchain, or just be a client accessing the transaction records from other full nodes.
The nearest you can be to being a "maintainer" is to run a Full node. Though it is completely community service and it won't benefit you in any way. Though, people are indeed earning sats on LN by becoming LN nodes.
Three or four years ago, when there was a lot of BTC activity, there were times when transactions took several days to be completed. Now they seem to go right through. Does it all depend on rewarding the miners?
When we "send transactions", that data gets transmitted throughout the peer-to-peer network and gets stored by every miner and full node in its fast memory (generally RAM). This pool of transactions is called the mempool. Miners pick the most "expensive" ones which have the most fee attached to them and start processing them into blocks. If your transactions has enough fees attached to it, the miners will pick it instantly and it'll be finalized. In times when lot of transactions are happening, only the most expensive ones go through. This causes several "normally prices" ones to stay/ get stuck in mempool and not being put into blocks.
If the miners all quit and shut down all their computers, would the blockchain cease to exist?
The Bitcoin blockchain started the day Satoshi started running it on his 2008-era PC. If all the miners just shutdown, the blockchain will continue to exist in the thousands of full-nodes. The difficulty will take time to adjust and then it'll continue like nothing happened.
Is the blockchain built into computers that are running BTC Core as promulgated by the Devs at
https://bitcoin.org/.
Kind of, yes.
Where is the blockchain, and how does it work?
Which "work" are you refering to?
