"Do they have systems for exchanging them back with the original casino or do they sell them or just write them off and destroy them."

Some casinos run their own chip returns. Not many any more.
There are private companies that pick up foreign chips/tokens every two weeks. Have no idea what they charge. They make the runs and then return any chips/tokens that a casino refuses because they have been cancelled.
The returned chips/tokens are thrown in a box slated for destruction. Some casinos will allow you to buy the returns but it is an infraction of NV gaming regs. That is why I never post the name of a casino I got those little cancelled gems from.

When the private companies 1st started, I use to go through all returns after they came back. No use in going through racks and racks of stuff before it goes out. It did not take long for the runners to get wise to the hobby. Less and less came back as time went on. Now I go through racks and racks whenever I can find the time.

Good story: I once found 30 Caesars Palace $5 non-neg chips in the destruction box. TCR about $30. Some idiot floorman had OKed them for play on a BJ table. I told the cage I was taking them and forked up $150. Cage manager said I could not have them because the casino mgr had a hold on them. The CM told me he was laying for the customer that played them and was going to bar him until he coughed up the $150. I ran the guy at the slot club. He was a pretty good player. I explained to the CM, Geez if I paid for them then the customer would not be embarased or barred. AND BESIDES, it was the idiot floorman that OKed them. I got the chips! I wonder if it was the same floorman that OKed $200 in James Bond fantasy chips for play on the tables.