They are linked by chains: every cart has a coin-operated lock accepting a chain and a chain for the next cart to connect.

https://upload.wikimedia.org/wikipedia/commons/thumb/b/b1/Pf...

Put a coin in the drawer and close it, the chain of the next cart pops out the back and you can take it. On return, you push the chain back in, the drawer pops open and you get your coin back.

Store employees just occasionally balance cart numbers between the corrals, if there's multiple.