Comment by pbasista
20 hours ago
They are soldered in. Replacement itself can probably be done by a professional repair technician who is equipped with the right tools. But it is not a self-service job that anyone can do.
On top of that, the drives might be somehow paired to the motherboard. And I would imagine that pairing a new one with the existing motherboard may require some proprietary software that only Apple has.
These are only my assumptions though. People e.g. at Louis Rossmann's repair shop might know more.
Apple does not use common parts for its storage. People are used to M.2 drives, but those have the controller on the stick, and Apple has integrated the controller inside its M-series. So you first need to find compatible storage in the first place, and the only place it can come from is a donor Mac.