Aight that's most of the easy stuff if it doesn't even spin up... The front panel connectors can be annoying, stupid little pins and block connectors.

Rip

It should more/less be like legos, in theory. But it can be sensitive to things not being fully seated. My first motherboard, the 24-pin connector was ridiculously tight, so I pushed it in pretty tight and it still wouldn't latch. Wouldn't boot because of it. Had to really put my weight into it to actually get it to go. My current one, otoh, just slid right in. I think RAM is another place where it can take more force than you would think to push them in.

And then sometimes things just go wrong. If some part is defective, it can be hard to test without spares. RAM fortunately you probably have 2 sticks, so you can try with just one and then the other. If you have an iGPU, you can try booting without the GPU. But other parts, most people probably don't have spares lying around and can be harder to troubleshoot if its just not doing anything. Your motherboard probably has a power button on it, so you could try booting with that to make sure it isn't the case power button.

And then you have things like partial function. Found out months after I got it, my first motherboard model had a small defect... if it lost power for more than like a tenth of a section, it wouldn't boot without some weird ritual and some goat sacrifices. Fortunately, there was a reddit thread about that issue and I somehow came across when trying to troubleshoot.

TLDR; PC building is not something I would recommend for someone who doesn't have anyone who has done it at least once before to help them with it or be willing to bang your head on the wall trying to get things to work if they don't go perfectly at first.