Most personal computers are assembled from commodity components, and frequently have unused spaces and sockets inside, allowing manufacturers to build a variety of specifications from a limited inventory. Theoretically, this is correct. And a host of extra components can be added, internally or externally. Few people actually upgrade their PCs.
Some upgrades are beyond the capabilities of the ordinary user.
Others are costly, because the original component must be discarded, making the upgraded machine more expensive than one which had the higher specification in the first place. Moreover, upgrading one component is seldom enough. If you wanted to convert a road car for racing, you might have to replace not only the engine, but the wheels, the brakes, the transmission, the subframe, and the windows. Once you replace the processor, it may also be necessary to upgrade the memory, discs, disc controller, display and graphics controller. By this time, you might as well have bought a new computer.