iPhone artificially destroys every photo taken

Are you seeing the increased brightness only on the iPhone in Photos or also, when you are viewing the photo on another device?

There are three different reasons that might be causing the photo to look brighter, after you have taken it. It depends on the settings for the camera and the lighting conditions, when you are taking the photo.

• You might have set a photographic style, that is automatically applied by the camera. This can be turned off, see: Use Photographic Styles with your iPhone cameraApple SupportOfficial Apple Support › iOS. To prevent the photographic styles from interfering, set the Photographic style to "Standard", or do not use the "High efficiency" format.
• Your iPhone will automatically develop the photos as HDR photos, when the lighting conditions are problematic and there is no motion in the image. Adjust HDR camera settings on iPhone - Apple Support . HDR will brighten the dark backgrounds, increase the contrast in the shadows and highlights, and create more saturated colors. This cannot be turned off on the new iPhone models. But you could take the photos as RAW images, to be able to develop the photos yourself.
• You may have set the "View Full HDR" option in Settings > Apps > Photos. With this option on, the image will appear brighter in Photos on the iPhone, but look normal on other devices.

Could any of these apply?

Does force-restarting iPhone help?

Or resetting all iOS settings via General > at the bottom? Make iCloud and maybe also a local backup to the Mac before trying, just in case. Do NOT reset to factory defaults at least yet.

Has the phone had this problem since you first got it? You can get a full refund for your phone within the first 14 days if you bought it directly from Apple, even without stating a reason. Other sellers will have their own policies for what justifies a refund.

If it just started recently after working fine for a while...

Most of us don't see the problem you describe, so it may be local to your phone, either a newly developed fault or some setting or combination of settings you accidentally created.

Let us know when it started, whether it affects all photos or some subset (maybe those taken in low light or string light), and what specific phone model and version of iOS you are running. Perhaps the next volunteer advisor (or I) will be able to give you more specific advice with more specific information.