How do I "Avatars"?

1. You'll want to research "Avatars" after Power Armor 2.
2. Create and place an avatar somewhere.
3. Create and enter an Avatar Control Center (like a car with Enter by default).
4. Select "Control" next to the new avatar

How do I get back to my original body?

Look for the Disconnect button in the top left corner, or use the hotkey (default F2)

How do Avatar Remote Control Units work?

Avatar Remote Control Units (ARDU) work by having an inventory that you can put avatars into. They will show up in the control center GUI if they haven't spawned an Avatar, and will spawn one from their inventory if you control it. While an ARDU has an avatar spawned, you will see the avatar in the GUI instead of the ARDU.