Automatics don't have a clutch, they have a fluid coupler, torque converter. Think of it like 2 fans facing each other. The fan attached to the engine is always spinning and blowing on the second fan which makes it spin. Since there is no direct connection you can hold the brake and stop one of the fans.

These old Army Corp film reels are great for learning about mechanical systems. Here's a film about torque converters. https://youtu.be/leCEmJA0WsI