Fastest way to factory reset an android before selling it

Back up WhatsApp separately first if you use it. It has its own backup under its settings (Chats > Chat backup), and it isn't always included in the normal phone backup. After the reset, take the phone off your Google account's device list and remove any cards it had for tap to pay. Not strictly needed once it's wiped but it keeps things tidy.

Remove your Google account BEFORE you reset, that's the step that matters. Settings > Passwords & accounts (or just Accounts) > your Google account > Remove. On a Samsung remove the Samsung account too. If you skip that, the phone comes up after the reset asking for your Google login (factory reset protection) and the buyer can't use it. Then you get the angry message. After that it's Settings > System > Reset options > Erase all data, a couple of minutes. Pull the SIM and memory card out too.