Something that makes a video look amateurish, it's the phone trying its best to prioritise a 'clear image', but that means changing parameters mid-recording.
Now, this isn't bad, it's ideal for someone who doesn't want to lose the moment without worrying about choosing the right setting (imagine a parent recording their child's recital or soccer game). But the trade-off is that it looks choppy.
But if you're in a controlled environment, you can set a fixed exposure (balance between ISO, shutter speed and aperture), framerate, bit-depth, focus distance, colour temperature and microphone gain depending on your intent.
As an example, image you want to have a high-contrast image with a dark silluette of someone and a bright background like a sunset, the default phone camera app will try to guess whether you want to focus on the subject or the background, and will switching between the two randomly. With manual control, you can chose, whatch you want.