End of the jet bridge facing the airplane door.
Sitting in an airplane takes me to the same headspace I was in the previous time I was on an airplane - it feels like I pick up my train of thought right where I left it months earlier. So yeah I can see how the aircraft door is a significant threshold.
Because I'm a huge tech nerd, I've always wanted to take closeup pictures of the door mechanism as well as the junction on the end of the jetbridge. Just how all that works is cool as hell to me. Never have because I'm not sure if that's allowed and also don't want to waste the cabin crew and other passengers' time by loitering in the entrance area.
The definition of liminal
https://en.wikipedia.org/wiki/Liminal_space