From the moment you start the game, Diablo 4 has crossplay automatically enabled, meaning you can encounter and play with players from any of the game's various platforms (PS4, PS5, Xbox One, Xbox Series X|S, PC).
Diablo 4: How To Turn Off Crossplay
Crossplay is implemented quite seamlessly in Diablo 4, so much so that it can be hard to find where to turn it off. This guide will help you out!