Can resetting an iPhone 12 remove the existing passcodes to access it?

Give the phone back to whoever gave it to you and have them follow the directions here --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If they can't do this or are unwilling to do this, you are proud owner of a paperweight you can't use as a phone.

The original owner of that iPhone did not properly erase it for your use. While you can reset it, if the original owner had Find My setup, which there’s a good chance, it will be Activation Locked. That is a security measure and cannot be bypassed. Ask the original owner to Erase then Remove their iPhone from their Find My app so that will wipe their data and passcode and Activation Locked, bringing you to the Hello screen. Allowing you to set it up for yourself.

only if they remove the activation lock using their Apple ID and password.

Asking for a passcode means they have not done that

What to do before you sell, give away or trade in your iPhone or iPad – Apple Support (UK)

LD150 wrote:

or a very small chopping board.

I would not recommend using any device containing a lithium battery as a small chopping board.

If you should get "lucky" enough to cut into the battery, you might not like the possible fire, toxic smoke, and battery cell "explosions" that might follow.

Call it "the phone's revenge!"

Servant of Cats wrote:

LD150 wrote:

or a very small chopping board.

I would not recommend using any device containing a lithium battery as a small chopping board.

If you should get "lucky" enough to cut into the battery, you might not like the possible fire, toxic smoke, and battery cell "explosions" that might follow.

Call it "the phone's revenge!"

Spoil sport.

or a very small chopping board.

LD150 wrote:

or a very small chopping board.

Indeed!