All of the recent iPhone and iPad models allows to unlock your device with your face using Face ID, but this feature is missing in Apple Mac, so you still use the traditional way i.e by typing in your password, setting up your Apple Watch to auto-unlock your Mac, or, depending on what model of Mac you have, use your fingerprint with Touch ID. But the code reportedly discovered in the latest beta of macOS Big Sur by 9to5Mac suggests that Face ID could be coming to Macs at some point in the future.
The code in the beta release apparently references Apple's codename for the TrueDepth camera. The TrueDepth camera is what enables Face ID in Face ID-capable devices like the iPhone 11 and the iPad Pro. The code reportedly also has snippets mentioning the terms "FaceDetect" and "BioCapture," which seems to point to biometric facial recognition of some kind.
If the new Macs get the facial recognition technology, you will be able to use it for unlocking your device, log in to websites without a password just by using Face ID, only if websites support it or you can play with Augmented Reality apps.
However, it's still unclear on which Macs might get Face ID feature and when will Apple finally release it for Macs.