They do have cross-play, but as far as I know it's opt-in when playing with friends. Matchmaking will usually group you with others playing in a similar way.
Fortnite is now "Skilled-based match making, with forced cross-play". Doesn't matter what platform you are on, you are matched by your relative skill level. High-level 6-finger-claw iPad players (and some of them are really incredible!) will be in lobbies with medium-level console and PC players.
I think this is at least partly a generational thing. Kids and teenagers seem to be much more willing than my peers to play demanding games on mobile, and to play them in public.
It sort of makes sense, since anyone my age or older didn't _really_ have access to serious mobile games growing up.
I grew up while android/ipads were starting to take off and spent a lot of my childhood playing fruit ninja and minecraft pocket edition. Now I'm >20 and I don't have a single game on my phone. I'm not sure what changed but I just don't have any desire to play light/casual games anymore. On the bus I am happy to just sit there with noise canceling headphones and think. Then when I get home I'll spend an hour on a VR game.
Good point actually, but that almost (?) two decades wide gap between mainstream gameboy mobile playing and the iPhone was quite an opportunity to drop the habit. Well, there was Nokia snake.
It's a separate adaptation of the game engine, but your account, purchased items, friends list, etc all carries across, so the only thing that doesn't transfer is an ongoing game.
They are separate, partitioned instances of the games though, tailored for the format.