When all players hit "end turn" the host work on your changes aka commands and process these actions.
Read through it ... you will do it even as Emporer :)
To come back to the question
RGA happens after movement and after combat ... when your Rebel ships set Primary Enemy or Kill you fight the planet and RGA your own world. That happens too experienced commanders as well :)
Keep in mind Rebel ships are immune to NUK / ATT friendly codes from the planets. The Enemy need ships to prevent you from RGA his worlds.
Show Media Item - Thats a good question to introduce the Host Order ...