Depends. There are some prebuild vendors (usually HP, Dell, ... and that ilk), that use proprietary connectors/pinouts/cables on their motherboards, PSUs and so on. (Places that prebuild your PC from other manufacturers parts usually dont do that, but follow standard.)
This means, that processor, ram, and graphicscard usually are always salvageable (they have to buy from other vendors following standards) - but motherboard or PSU might not be, depending on where you are buying from.
Some of the mentioned vendors also limit the grafics cards you could use within that system bios side. And more restrictively than a normal motherboard vendor would.
If you are not buying from said brands above, but rather from more 'boutique' system builders this isnt an issue.
Also for power pinouts, there are converter cables (sold on aliexpress), but getting them (or making them yourself), and knowing that you'd have to get them and... is a pain..