ELI5: How do casinos keep track of the amount of chips in a dealer's care at a table to make sure the dealer isn't pocketing them or paying out too much?

I used to think it was some kind of process like a cashier till where the dealer would pick up a tray with X amount of chips, and then have to turn it in where everything is counted and reconciled somehow.

But I saw in a movie (never been to a casino myself to gamble) where one dealer would just walk up and tag-out another dealer and take over their spot. It made me wonder how everything was accounted for.

What's to keep a dealer from slipping an extra $500 chip every once in awhile to their buddies?