And here's your other issue...
Nothing of what I said was meant as an offense. Yet you took it to hearts. That is not a response one would normally expect. It's almost as if I'm reading a script from a teenage drama show where a character has an unresolved inner conflict. Doesn't mean I'm correct, just some food for thought.
Your question was "why people act like I'm a kid?" I only shared my observations of why that could be the case. That doesn't mean you need to change anything. Not if you yourself are ok with it.
Also, I was well aware you're 21 at the moment of writing the comment. Yet again, I'm merely answering your question, I really don't mean anything beyond what is written. I'm not judging, no nothing, everything I write should be read in as neutral tone as possible.
The rest was me speculating. Those aren't the questions you should be answering to me, only to yourself. After all, a random stranger on the internet can only do so much, you're the only one who can answer your own question. Best I can do is point you in a direction to dig further, which those were meant for. It's all about retrospective.
It's quite the opposite, though. PC components have never been as compatible as they are today with the inclusion of different standards like ATX and stuff.
As for you figuring stuff out, here's how I pick parts:
Coolers: I go see the temperature tests to decide on which one fits depending on noise vs temps vs price.
Motherboards: Here are the main bits I look at
Then there's extra
PSU: Very simple, go to power draw calculator or multiply power draw from pcpartpicker by 1.3 or 1.4, that's your Wattage requirement. Then find a list of reliable PSUs, look for cheapest reliable one that has enough Watts. It's a good idea to have some overhead as well. Alternatively to a tier-list is knowing which manufacturers are good.
Cooling for RAM: ignore cooling for RAM, not important at all. It's mostly for looks.
RAM clock speeds: MT/s, aka Mhz, is bus width. Higher amount = more data can pass at once. But we're currently at a point when 6000mhz doesn't make much difference against 3600mhz. So, latency is more important. Google, which combination of clock speed + timings (they look like 36-38-38, can also be written as CL36) has lowest latency, go with lowest.
Pcpartpicker makes sure things you put together are compatible with each other. So, start with CPU and GPU.