Why can't you play multiplayer between Samsung and iphone?

Not yet but they may add that in a future update

You can you just have to press play then press multiplayer then you press the small rectangle then you host a lobby then you pick a map then once you get in the map there will be a code and the other person will do the same thing as you did b it they won’t press on the host button so what they will do is they will type the code in then you can play but if it does not work then somebody will need to update off-road outlaws but if it still doesn’t work after that then Idk what you would do