This amazing piece of gearing has appeared on HowStuffWorks before. You could understand it from the electric screwdriver article. An automatic transmission contains two entire planetary gearsets folded alongside one another into just one part. See How Gear Ratios Work for an introduction to planetary gearsets. The guide transmission is https://jaredbcyuo.bloguetechno.com/rumored-buzz-on-gearbox-auto-69248183