To add to that point, you can't build your own laptop because mobile components are not affordable and they cannot be bought individually. Also, most components of the laptop are soldered to the motherboard, although some laptops have socketed CPUs and SODIMM slots as well. But hey, you don't need to waste your time putting the components in - the manufacturers will waste their time for you.
Custom PCs and prebuilt PCs are just built the same way, with the same components and stuff that a computer needs. The difference is that who will build it.