A lot of insightful comments already, but there are two other tricks I think Apple is using: (1) the laptops can get really hot before the fans turn on audibly and (2) the fans are engineered to be super quiet. So even if they run on low RPM, you won't hear them. This makes the M-series seem even more efficient than they are.
Also, especially the MacBook Pros have really large batteries, on average larger than the competition. This increases the battery runtime.
Also, especially the MacBook Pros have really large batteries, on average larger than the competition. This increases the battery runtime.