Apple

Not if you reset the phone to stock before returning it

Potentially yes, but through social engineering. Guy goes into a carrier store with your SIM card, and says “I’ve just been robbed, they took all my devices, my wallet, and my ID, but I still have my old SIM card, can you help?” And there’s a chance the people at the store might consider that enough verification to prove ownership. I dunno how common that is, but to provide some peace of mind, as long as you aren’t famous or some dignitary, you should be fine. Most theifs aren’t going to go through the hassle of stealing some randos account.

Your old sim will be deactivated when you activated e sim on your new phone. Worst they can do now is put the old sim in another phone and get your phone number (but do nothing with it)