iPhone Camera works on all other apps except the actual camera app

If the Camera app itself shows a black screen for the rear camera, but apps like WhatsApp, Instagram, Zoom, or other third‑party apps can still access and display the rear camera, that strongly suggests an iOS or Camera app software issue rather than a hardware failure of the camera module.

Check after each step: Steps to Troubleshoot

1. Switch Camera Modes
   1. Open the Camera app.
   2. Swipe between Photo, Video, Portrait, Pano, etc.
   3. Check if any mode successfully activates the rear camera.
2. Test Different Rear Lenses
   1. Tap 0.5×, 1×, and 2×/5× (depending on your iPhone model).
   2. If one lens works but another stays black, the issue may be isolated to a specific module.
3. Remove Accessories
   1. Take off magnetic cases, clip‑on lenses, or other attachments.
   2. Rarely, these can interfere with camera initialisation.
4. Force‑Close and Relaunch Camera
   1. Open the App Switcher.
   2. Swipe the Camera app away.
   3. Reopen it to see if the feed restores.
5. Reset All Settings
   1. Go to Settings → General → Transfer or Reset iPhone → Reset → Reset All Settings.
   2. This won’t erase photos or apps, but it will reset system preferences, which can clear out conflicts.

You say all other apps. Are any of those other apps using the rear cameras? Sandmarc Camera is a free camera app. You can download and try it.

https://apps.apple.com/us/app/sandmarc-camera/id6759989969

I guess this is caused by some kind of hardware damage. Has the device been flooded, damp or dropped recently?