Additional features, settings, or issues not covered by specific Microsoft Teams categories
I had the same problem, and then I discovered how I didn't give Teams permissions to access camera. Installer should probably ask for this, but it's likely via a dialog that that installer doesn't control, so it's often open behind all other windows, and then it gets ignored and eventually closed or auto-closed (e.g. upon reboot) - and then, of course it ain't working...
So, System Preferences > Security & Privacy > [Unlock] > Select Camera within the list on the left > make sure checkbox for Teams is selected within the list on the right.
Probably double-check the same for the microphone.
Then, you'll have to quit Teams for this to apply. Then, start Teams, and within its own settings > Devices, verify that camera preview is showing your phace :)
Good luck!