I would like to play with a friend who has the Windows 10 version of the game on the PC. I heard that crossplay is possible with the mobile version, but only when you are on the same WLAN. Is that true or can you simply join your friend via the Xbox app without being on the same Wi-Fi?
You can add a friend with Minecraft by clicking on "play" then on "friends" and then on "add friend" then you have to enter the name of the player and then you can add him. He will automatically become your friend at X-Box. Then you can open a world and then hit pause and send the friend an invite into the game, then they can join you.
