When toggled with "R" the PlexDoor will open, and can be hooked together to make giant doors for hangers or air locks. In build mode, simply place these down next to eachother to create a door. They will automatically link together and do not need anything else to function.
The PlexDoor blocks are buyable in the shop and offer a low stock in most of the shops. If you are planning to create a big ship with big docking stations, you will need to travel to several shops to get enough doors.
Doors within a faction can also be opened by firing the Docking Beam at them. If you are trying to dock in your base past doors, you will need to physically move your Ship Core past the door, else its beam will activate the door instead of docking. This makes your ship stuck in the middle of a closing door, and ejects the ship into space.
These doors work well for your home base of your faction. If the only way in or out is through doors that are kept closed, as the faction home cannot be destroyed they add significant defenses to your base.