Affirm at Apple Store?

I’m somewhat new to affirm. I have used it only on Amazon in the past. I really want to get a new iPhone. I’m wondering if anyone can walk me through how I can use my affirm card at the Apple Store (if even possible) I would like to purchase the phone in person because I want to trade in my old phone. I assume it’s possible to do so online but I am not sure how the trade in works (plus my purchase power might not cover the total amount of the phone without the trade in) thanks in advance for any advice!