If I want two users to chat with each other instantly inside my Unreal Engine mobile app, what technology or method should I use?
If it’s just between players in the same game, you can build a simple chat system using RPCs. If you want an actual dedicated chat system though, you’re probably better off running a separate server for the chat and using existing chat technology such as XMPP or IRC. Since Unreal has built-in XMPP support, that would be my first choice, though the built-in XMPP support isn’t very good (at least as of 4.27) and is full of holes. Still, it gives you a base to start from.