Hi danai260,
Thank you for using Apple Support Communities.
We see that you have an issue with your iPhone 12 dimming and going to a dark screen when you are on a FaceTime or WhatsApp video call. We want to help. Have you tried restarting and checking again? If not, here's how Restart your iPhone
If a restart doesn't help, is your iPhone updated? It's a good idea to back up your iPhone before updating.
Here’s How to back up your iPhone, iPad, and iPod touch - Apple Support
Here’s how to Update your iPhone, iPad, or iPod touch - Apple Support
Reach back out if those steps don't resolve the issue. We would love to help.
Best Regards!