Mongo Playground

Configuration

db={"users":[{"userId":1,"username":"Naruto","displayName":"Hokage","createdAt":"","avatarUrl":"","stats":{"followers":[2]}},{"userId":2,"username":"Luffy","displayName":"King of the pirates","createdAt":"","avatarUrl":"","stats":{"followers":[]}}]}

Query

db.users.aggregate([{"$lookup":{"from":"users","localField":"stats.followers","foreignField":"userId","as":"stats.followers.userData"}}])

Result