Lens cleanliness is the first thing you should check, because even microscopic fingerprints can cause that exact kind of haziness. Second - make sure you're not accidentally covering the camera with your finger while shooting, that happens a lot with side-facing shots for Shorts.
If the lens is fine, try shooting not in the YouTube app, but directly in the iPhone's standard camera app with a higher video resolution format, and then upload the finished video. The YouTube app often compresses the quality already during recording, whereas the native camera preserves more detail. One more tip - shoot from at least half a meter to a meter away, the iPhone focuses unstably when you're too close to your face and you get blur.