Hey there, Nosssa!
From your post, it sounds like you are experiencing an issue with the camera on your iPhone 12. We’re glad that you reached out to the Apple Support Communities. We want to try and help you find the answers you need.
To verify, what version of the iOS software do you currently have installed on your iPhone? You can find this in Settings > General > About. If you have not tried already, please Restart your iPhone. Does this help to resolve the issue? Often this simple step will refresh the apps and software on the device to solve numerous problems you may encounter.
Does the issue happen within all apps that use the camera, or only the built-in Camera app? If other apps, which other apps? Also, when you have a moment, go through these steps: If the camera or flash on your iPhone, iPad, or iPod touch isn't working.
Take care!