What are multi-platform projects in the Playstation area?

Can you no longer play this alone, but only with several participants? Or is it no longer for the ps4 and ps5, but only for Xbox, computer Steam?

That would be really stupid for my cause if a continuation of my DLC story (Detroit become human) would no longer come out playable alone. 😪

Multi-platform simply means that it is not only coming out for one console but for all platforms.