Face recognition
Why can other people use my phone and unlock it with their face
iPhone 12, iOS 14
They can't but they love having fun making you paranoid about it. Unless they are a one in a several million match to your face/facial structure they cannot open it.
The True Depth camera has quite a range so if you are standing beside or behind this fun person when they show they can open your iPhone it is actually seeing you and opening. If this fun person knew your passcode, or you let them use your iPhone while it is unlocked, they could have set up an alternate Face ID entry.
So here is the test: erase all face ID entries and reenter yours, change your passcode to a 6 digit one that this person would never guess. Lock your iPhone with the side button, turn it face down, hand it to the person and have them go into another room. Tell them to open the phone, open Pages, type "I can open your iPhone", insert the date and time, and bring it back to you.