Accounts that inflate a follower count without being real potential customers. On platforms that expose follower lists they can be sampled and estimated. On YouTube no such list is public, so any percentage quoted for a YouTube channel is invented.
Whether an audience behaves like a real one: what share actually watch, whether engagement matches the platform norm, whether views are steady or spiky.
A group of creators who agree to like and comment on each other's posts to raise engagement rate.
Automated accounts generating views or interactions.