Hello Paolo_M,
Thank you for using Apple Support Communities! I understand that the built-in camera is not working in multiple apps on your Mac. It's important that you're able to connect to and use your camera when you need it. I would be happy to help point you towards a resolution.
Let's start by going to System Preferences > Security & Privacy, select the Privacy tab, then select Camera. Ensure that camera access is enabled for the apps you would like to use it with. If there's no change, testing if your camera works in another user account could help us isolate if this may be caused by user-specific settings. If you don't have another user account, here's how you can create one for testing:
How to test an issue in another user account on your Mac - Apple Support
If the camera does not work in a different user account, use the System Information app on your Mac to see if FaceTime HD (Built-in) appears under the USB header:
About System Information on your Mac - Apple Support
If your built-in camera doesn't appear, try to reset the SMC on your Mac, and check again:
How to reset the System Management Controller (SMC) on your Mac - Apple Support
You may also want to consider using Apple Diagnostics to see if it detects any hardware issues with your Mac:
Diagnose problems with your Mac - Apple Support
Let us know what happens after trying these steps.
Cheers!