While purchasing a PC, one gets a cookie-cutter PC. One can select such choices as a bigger HDD, greater memory, or a better screen, but overall you get what the retailer supplies. If you want s a better power supply or quieter cooling fans or a motherboard with additional features, they are not available.
Everything available is a matter of choice. Big direct sellers such as Gateway and Dell frequently use multiple sources for parts. 2 similar systems ordered on the same date containing significantly various parts, having crucial differences as various motherboards or screen with the same model number but of different manufacturers. While building a PC, you get to decide what to add and what to leave.