My Garmin watch has Garmin Pay, which works even if I don't have my phone with me. I think I just presumed that there is a bank balance cached somewhere in the app so that an attempt to spend up to a hard limit or that amount will work.
Take a debit card for example. It's connected to a bank account, and cannot give credit. Garmin must be taking some of the risk on board, since the bank or card provider do not.
There's also the fact that anecdotally people are reporting different spend limits. On a thread I saw one person claim that £30 was their limit and another saying "My largest payment so far on the watch has been £275"
There can be other limits in place (like those controlling whether or not a PIN has to be entered), but those are on top of the requirement to obtain online approval from the issuing bank.