Rule 1: The DM describes the environment.

Rule 2: The players describe what they want to do.

Rule 3: The DM narrates the results of the adventurers actions.

The rules on how to play are pretty basic and very flexible. You can easily play an entire game or campaign not going past those three rules or needing anything more in depth than that.

Some folks like crunch. Some folks like heavy crunch, some like light crunch. Some folks just want a mushy bowl of cereal. We're all just playing make believe. What rules you use is up to your table and what kind of crunch they want.