For many people, Face ID has been frustrating to use for the past two years because it doesn't work with a face mask, but that's changing in iOS 15.4, an update that's currently available to developers and public beta testers.

MAsk On Face ID iOS 15
iOS 15.4 adds Face ID with a Mask, letting you unlock your iPhone with Face ID even when your face is covered, so Face ID is as convenient with a mask as it is without one.

How Face ID With a Mask Works
With the lower half of your face covered by a mask, Face ID scans your eye area to authenticate your identity. According to Apple, Face ID with a mask recognizes the "unique features" around the eye area rather than looking at the entire face when you're wearing a mask.

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Face ID when wearing a mask works just like Face ID without a mask. You swipe up on the ‌iPhone‌'s display and it unlocks with a Face ID scan. Face ID with a mask also works interchangeably with standard Face ID, so your phone works in the same way when you're wearing a mask and when you're not.
Face ID with a mask is identical to standard Face ID in almost every way, but there are a few additional setup steps to go through, and it does not work with sunglasses. There are also a few more limitations with unlocking angles because Face ID with a Mask needs to clearly see your eyes before it will unlock.

https://www.macrumors.com/guide/mask-face-id/