What is an app niche?
An app niche is one job people go to the app stores for — scan a receipt, identify a plant, track a habit — together with the apps that already answer it.
A niche is defined by a search, not by a category. “Photo & Video” is a category; “remove background from photo” is a niche. Each niche has its own leaders, its own prices and its own complaints, which is why two niches in the same category can be completely different businesses.
Because the stores are search engines, a niche is usually named by the phrase people type. The top ten results for that phrase are its competition.