How do you allow the player to freely transition between levels/scenes?

So I know about load level and load level async but I do not believe the answers I have read answer my question. I have a scene/level that is the main world, on the main world I placed a bunker door that I want to use and allow the player to go to the scene/level that is the underground bunker and return. In short I need a trigger/interface on the doors (inside/outside bunker) to allow this transition to happen freely without destroying everything in the main world and ideally without having all scenes/levels loaded all the time in the background.