garmin pay, you need to enter passcode for lower-value transactions as well?

Yes. But only once a day if you don't take the watch off.

That's how it's more secure than carrying cards.

Same with the Fitbit pay.

The pin-code entered on the watch is for unlocking the wallet on the watch and get access to the card(s) on the watch. You never have to enter a pin-code on the payment terminal, it is already authorized since you needed a pin on the watch to open the wallet. This is pretty much how every electronic wallets work (Samsung, Apple, Google, Fitbit...).

As Adam mentioned the watch that will keep the wallet open for 24h unless you take the watch off your wrist (detected using the OHR sensor).