There will be standard PCIe v4 SSDs with a Sony approved logo on them. Meaning they will be fast enough and will fit in the compartment, but will not be proprietary (unlike Xbox ones). Also I imagine that some of the games, at least in the first years, will not necessarily require to be installed on the SSD and could work fine on an external drives (with HDDs or SSDs on the inside). Personally I do not tend to play more than 2-3 games, so I’m not worried about the capacity. I do use external drive for my PS4 (I have only a 512G SSD in it) as a storage for games. Copying them over is faster than re-downloading and should be even more so on the PS5.