There are two types of automatic doors - sliding and swing.
Swing doors operate via a mechanism mounted above the door on the door jamb or architrave. The mechanism operates an arm which is attached to the door, either pushing or pulling it closed. The mechanism is a spring, with hydraulic fluid (oil) regulating the speed at which the spring closes the door. These closers are relatively small and need human force to open the door.
This mechanism is automated by an electric motor either pumping the hydraulic fluid to open the door, or by opening it directly. The mother is actuated by movement sensors or push buttons, or a security interface.
Automated swing doors must allow egress in the event of power failure.
Sliding doors are mounted on wheels which slide on a track above the door opening. They are coupled together with a belt or chain, and a motor drives the belt or chain, opening and closing the door. The door is locked either manually or automatically with a magnetic lock attached to the belt or chain. They can be activated by movement sensors or push buttons or a security interface.