Refers to the opening or entrance itself, often with a door or hatch. Used for both architectural and vehicle contexts.
船の扉口から貨物を積み込んだ。
We loaded cargo through the ship's hatch.
古い城の扉口は狭くて暗かった。
The doorway of the old castle was narrow and dark.
Compound of 扉 (とびら, 'door') and 口 (くち, 'opening'). The reading とびらぐち is a straightforward combination of the two components.