That is essentially correct, except for having to finish the game in one session.
If you continue playing with the same person (presumably the most common situation), you would be able to use the same main character (and companion(s)) in subsequent play sessions. When you join you would take over a character from the existing party, and the host can reassign characters if required.
If you were no longer able to play on the same schedule, the host could send the latest save file to you, and you could each continue separately.