Can’t turn off iPhone 12 .. tried to volume up down and side button without response

Per the Related Article, linked above… Quick press up-volume, quick press down-volume, quick press and hold the side button until after the Apple logo appears. Continue to hold the third button press past when the sliders appear, if those should appear. Wait for the Apple logo. If you have a protective case, temporarily remove it while pressing the buttons. If that three-button sequence does not restart the iPhone, contact Apple Support.

Often users who try the Forced Restart do the Volume Up quick press and Volume Down quick press OK, but when they hold the Side button down, they release the Side button when the Power Off "slider" appears.

That is wrong. You have to keep holding the Side button until the Apple logo appears in the center of the screen. This might take 20-30 seconds.

When the Apple logo does appear, then you can release the Side button to let the iPhone restart.

If the Forced Restart does allow the phone to return to normal operation after a few attempts, then there is a hardware problem with the phone that will need attention at the Apple Store....make an appointment....or visit an authorized Apple service shop.