Hard factory reset using buttons on iPhone 12

You cannot factory reset your iPhone with only the buttons as that is a multi-step procedure requiring you to enter your Apple Account credentials. If you mean a Forced Restart, which reboots the phone but retains data, you can press and quickly release the Volume Up Button, press and quickly release the Volume Down Button, then press and hold the Side Button until you see the Apple Logo.

nikko297 wrote:

Tried but for some reason didnt work ? How long am I supposed to press for?

For the first two buttons, press and release very quickly, less than a second. For the third button, continue to press until you see the Apple Logo which can take up to 30 seconds. Let me know how this goes for you.