This photo double was for a 12 year old kid. Minors can only work so many hours a day, so they have someone dress like them to film all the shots that aren’t close up so the main actor doesn’t have to work as long.
And I follow the facebook pages for the extras casting agency in my area. They post what kind of people they need and if you fit, you just send them an email and if they want you they let you know. You don’t need any experience or anything. :)