HelloMy Mazda 2 (2020) is almost every ride is warning for center rear seat belt, with no one in the seat.
my dealership cant find anything wrong. Please helpThe dealership says it does not exist any senders in the seat. How does it work and what can I do to fix it?

 

Welcome to the forum
For a DIY ...
Under seat disconnect the harness(s) to the seat, start the engine for about a minute then turn off. Reconnect all the harness under the seat and check. If seat belt light is still on, connect the belt as if someone was in the seat and start the engine. The seat belt light should go off if not then the seat belt latch or sensors may be faulty.
You may need to clean using electrical parts cleaner only all the contact areas hearnes and belt laching.
Report after you do this check procedure.

 

There is no sensor in the seat, there is however a sensor in the seat belt buckle.

 

generically speaking on some service suggestions ...I was working on a early 2nd gen PT Cruiser that day the guy had the same issue. as the OP here. Only his dash alarm was going on constantly ding ding ding ding... LOL His was both seat and buckle that he had spoiled a Large coke and it dried. Easy fix just cleaned both connectors and latch area. I had to wear a mask and gloves working on it because of all the crap under the seat that had been going.... LOL