Replacing the extra internal SSD (the one you need to install). I know the internal SSD (that came with the PS5) is soldered on the PCB.
You need to replacing when your 1 or 2T SSD (that you installed) ran out of room.
For XBox you just swap card (5 sec). You buy 3 1T cards, you get 3T storage. For PS5, you buy 1T storage, then you buy another 2T SSD, you get only 2T, The internal extra SSD slot is not design to be swappable.
Just think of it like the XBOX external card is USB sticks, while the internal SSD card is like a harddisk.