How do I use the charging lil circle thing. I put it the back of the phone and it don’t charge the phone

Re: "how do I charge the phone with a small circle magnet charger"

Learn about charging with the MagSafe Charger and MagSafe Duo Charger.

or

Apple Support's online article: How to wirelessly charge your iPhone - Apple Support

says:

Charge wirelessly

1. Connect your charger to power.
2. Use the power adapter that came with your accessory or a power adapter recommended by the manufacturer.
3. Place the charger on a level surface or other location recommended by the manufacturer.
4. Place your iPhone on the charger with the display facing up.
5. For best performance, place it in the center of the charger or in the location recommended by manufacturer.
6. Your iPhone should start charging a few seconds after you place it on your wireless charger.

You should see  in the status bar.

If needed: Check the above article for tips or trouble-shooting.

How do I use the circular magnetic device to charge my Apple series 9 watch