If you want to add a pair of doors to your level just create the Door from the Level Design Tool , ensuring the arrow is aiming at direction the player will be looking when entering the room.
Each door has to be set as an Exit to each other on their respective SceneDoor component. If a door has an exit it’ll show a red line between the doors. If two doors have each other as an exit a green line will be shown connecting the doors.
Doors do not have any visuals by default. You can of course, add any model you want to represent the door and create prefab variants or other prefabs that contain the existing Door prefab with specific visuals.