How to activate Face ID on my iPhone 12

If Face ID isn't working on your iPhone or iPad Pro - Apple Support

With iPhone 12 models and later using iOS 15.2 and later, you can go to Settings > General > About to see the parts and service history of your iPhone and you can see if the camera has been replaced. If the repair was done using genuine Apple parts and processes, you will see "Genuine Apple Part" next to Camera.