How does iPhone trade-in policy work?

I've always ordered online and was charged the full amount and then they credited the card I used to pay after the trade was received and approved. However I just checked Apple Trade In - Apple

and found there is another possibility. See the following from that support article:

If I buy a new device online with a trade-in, when is credit applied?

If you pay in full, we’ll credit your original payment method after we receive your device and verify its condition. If you pay monthly, your credit will be applied to your monthly bill.

Yes. They charge your credit or debit card for the full purchase and then credit it back when the trade is received and approved.

Also see the following for other trade information: Apple Trade In - Apple

• edited. Better answer earlier.