I've gone through 2 cycles of being on the iPhone Upgrade Program (IUP). It is through Citizens bank. You're basically taking out an interest free 24 month loan for the amount of your phone. You pay for taxes up front. You can make payments to Citizens on your Apple Card and Apple will give you the 3% Apple Cash back. Then, after 12 months/payments, you're eligible to upgrade to a new phone. Buy the new phone, send/take in your original one. The difference between the IUP and the new 24 month interest free loans on your Apple Card is that 1) the IUP price includes 2 years Apple Care+ in the monthly payment (no savings, it's just required) and 2) you're guaranteed a "trade in value". Example: I bought the 11 Pro on the IUP last year. I'll have $550 left on my loan when I trade it in for the 12 Pro on 10/23. So in essence, my trade in value for the 11 Pro is $550. If I had purchased the 11 Pro on a program like the 24 month Apple Card plan and wanted to sell it to buy the new 12 Pro I, 1) wouldn't have been required to include Apple Care+ and 2) would be looking at resale in values of ~$420 (looking at third party companies like Gazelle).
If you want to do the IUP program, you can get pre-approved for it (remember, it's basically taking out a loan) by starting the buy process in the Apple App. Once you selected the model, etc, it will ask if you want monthly or one time payment. Choose monthly and the IUP will come up as an option (note: it seems like this doesn't show up as an option if you choose the SIM free model). Choose that you want to newly start the program and it will take you through the page to get pre-approved. Then, you'll all ready to make your purchase on pre-order day. This is the confirmation I got once I went through it