Is Helldivers 2 cross-platform or only on PC?

Yes, Helldivers 2 has cross-platform play: users can play together even if one is on PC and the other on PS5. If you want to play with your mate on a different platform, go to the Social tab, enter your friend's code in the Search field, and add them to your friend list. Alternatively, your friend can do the same with your code, which can be found at the bottom of the same Social tab. After adding your friend, choose Invite to Squad or Join Squad to start playing together. The only downside is that Xbox owners can't join the game - it simply hasn't been added (and likely never will be) on Xbox.
https://youtu.be/ftk6c8eh8yA