The SSD modules for 2019 Mac Pro have no controller on them, they're just raw capacity and cannot function on their own at all. The controller is T2. The modules are directly accessible in Mac Pro, easily removable with screws (no tape to be sliced), and officially available as an OEM part from Apple. Setup requires the modules to be wiped entirely and setup must be done with Configurator 2 through another machine.
This is not the case with Studio at all. One could argue the modules are not even meant or designed to be accessible. It's just cheaper for Apple to BTO with module(s) in this manner to better meet custom order specs. If it were cheaper to solder the NAND onto the board, they would do that instead. They clearly alert the customer storage is what it is when purchasing and no expectations of capacity increases.
Good luck getting 3rd party modules that must be setup through T2 and firmware to work without support through Configurator 2. It's not like a third party can just make an adapter and use off the shelf NVMe blades like some previous MBP's could.
