The PS5 has an 825GB PCIe 4.0 SSD soldered to the motherboard. It uses a proprietary custom controller.
There is a user-accessible slot for a m.2 2282 PCIe 4.0 SSD under the left side panel for future upgrades.
Currently, any attempt to add a NVMe drive results in the following error:
A module is inserted in the expansion slot. Turn off your PS5, remove the module, and then turn the PS5 back on. Your PS5 will shut down in 30 seconds.
(Tested on a v1.00 CFI-1015A w/ a Samsung 970 EVO Plus)