NOOB ALERT:

I am somewhat new to cryptocurrency and I have many questions.

I somewhat understand how mining works. My understanding is that computers are used to process transactions to make sure that someone doesn't use the same coin twice or change one coin into ten. So from that I am able to deduce that the more people use cryptocurrency, the more miners are necessary to process all of these transactions. That brings up a question. If everyone were to, all at once, stop using cryptocurrency, then would there be nothing for the computers to mine?

Now moving onto the hardware side. My friend has a GTX 1080 ti and I recently persuaded him to use it to mine. The rate estimate that he was getting was anywhere from $3.00 - $4.20/day. Now $0.12 per kWh and that's anywhere from $750-$1000 profit per year, and about $2000 in the lifetime of the card (what I understand is that a GPU mining 24/7 will last about two years). That means that by the time his GPU dies, he can buy two more. Now I know that NVIDIA doesn't make the best GPU's for mining. From what I understand, are the RX480/580 cards the best for mining.  So why doesn't everyone just mine if its so profitable? I feel like there is a catch somewhere but I am just unable to find it. That brings up another question. What the heck is up with all the "hash" types? Like what in the world is Sols/s and Equihash? I thought that it would all be one unit of measurements, such as Hash/s. From what I know, hash/s is sort of like a GHz: the speed at which you mine or process. So why are there so many different types? I have also heard about ASIC cards. They show that the cheapest one goes about 1.115 TH/s (idk which hash type that is and how it converts to others). Coming in at $139 and being more power efficient than a PC why doesn't everyone just use those?

I have also seen pictures of racks of GPU's from floor to ceiling and I can't help but wonder how they connect all of those to a motherboard. Is it just one specialized motherboard, or is there one motherboard for every 4-5 GPU's?

Sorry that I asked so many questions but I am trying to get into mining and I am willing to spend a few K and I just don't want to make any mistakes that will cost me greatly in the future. What GPU is the best to use? I heard that for some coins the RX 480 is better but for others the RX 580. Is there any difference in performance between the 4GB variant and the 8GB variant? Sorry if I sounded dumb and uneducated. Thanks a lot if you take the time to respond.