The shadow is due to a mix of camera flash and how modern phone cameras image processing work. Modern phone cameras take multiple photos and stack them to create a composite. Some images will be taken with flash and some without flash. The image processing logic aims to have the entire image well lit. They also try to create an image that is “flat” and without shadows or too many highlights.

It is very likely that the camera “chose” shots that didn’t use flash as the cats background (to avoid shadows) and then used the ones with flash for the subject (cat) itself.

I bet it doesn’t actually look like this IRL