To improve the quality of your footage switch your iPhone camera to shoot in RAW format. By default, iPhone cameras compress the file size of images by saving them as a JPEG. But by switching to RAW format the iPhone can maintain a higher resolution.
Not every picture taken on an iPhone should be shot in RAW format. Otherwise, the all the iPhone storage space would be wasted solely on saving photos. Using RAW format is only necessary when the image is going to go through post-production editing. The RAW file will allow for the picture to be edited without compromising its quality.
The only downside to shooting RAW footage on the iPhone is that the camera can no longer use computational photography and machine learning. The iPhone 12 Pro and iPhone 12 Pro Max were both designed with new ProRAW technology which for the first time allows the iPhone camera to shoot raw footage while still using LiDAR, Smart HDR, and Deep Fusion.