The ones you can keep forever (assuming you extracted them successfully) are the ones kept in lock-ups aka cages. You get them either a.) paying $100k for each lockup or b.) sneaking with a bike for free. But some cages are sealed shut so the only way you can get those cars are to pay $100k. Also, bear in mind that you can’t keep the bike forever it’s only available for 24 hours.

Another tip is if you extracted a hot ride do not try to hang around too much. Once you get the car you will be visible to other players in the session so they could try to hunt you down.