If it's via the Apple Upgrade Program, it's essentially leasing. You take out a loan with Citizens One, and the phone is paid for (from Apple's perspective) ou

If it's via the Apple Upgrade Program, it's essentially leasing. You take out a loan with Citizens One, and the phone is paid for (from Apple's perspective) outright. You then pay the loan down. However you can't get another upgrade until you've paid at least half of the installments, and then if you trade in the old phone the remaining balance of the loan is deleted. There's also a new (similar) program integrated into their Apple Card. Not sure how exactly that works. The inventory comes from Apple. You do not buy/lease/whatever the phone from any carrier. Once you purchase it, you have the option to activate it with whichever of the big carriers in store. You could also walk out and install your own SIM from elsewhere. AUP phones aren't carrier locked.