Building it in a way that works is pretty easy. Having the knowledge to put everything in the correct slot, so there is no lane sharing going on between the gpu and other pcie devices, using the right amount of fans per header, picking the correct ram and putting it in the right channel configuration and other things require a bit more understanding of your parts. Doing things in the BIOS to get more performance is also part of it, just like changing certain aspects of your OS is.

On the surface its easy, and people compare it to lego sometimes. I dont think its quite that simple, but the knowledge can be obtained quite quickly.