In principle, Cross Save means that a storage device is backed up and distributed via various devices and platforms. In the optimal case: On Playstation, a character is raised to level 5 and can continue to be used in Stadia.
It should be noted that Cross Save does not mean cross gaming or cross DLCs. Playstation players are unlikely to be put on a server at the same time on the other platforms.
A DLC bought on the Playstation is probably not available on the other platforms.