How to factory reset an iPhone I sold?

I recently sold my old iPhone 12 due to it having water damage and the screen eventually just became black all the time even though I could still hear all my notifications, calls, etc coming through, just had a black screen, so therefore I couldn’t manually factory reset it prior to selling and needed the money to buy a new one. The buyer just msged me and said they fix it and asked me for my passcode, which I’m refusing to give until I can figure out how to erase all the content first myself.

[Re-Titled by Moderator]