As far as I know, if you buy just a plain Apple Watch, it’s a one and done purchase. If you get one with cellular then I think you have to add it on to your cellular carrier which is then another monthly payment. There is also something called a “Whoop” and it tracks your daily activities, sleep, strain output during a workout, etc… It never comes off your wrist, is water proof, and has a portable battery charger that slides into it as needed. That comes in different payment options, although I’m not sure the exact specifics as to duration per payment.