door bar placed outside a door
扃 refers to a horizontal bar used to lock a door from the outside, essentially a door bolt or latch. Its meaning is specific to this physical object and its function.
扃 combines 户 (door) with 冋, which likely contributes a phonetic element. The character represents a door bar or bolt used to secure a door from the outside.
The outer 户 is a door, and the inner 冋 looks like a bar slid across it. Picture a heavy wooden bar placed outside a door to lock it shut.
For ケイ, imagine a key (ケイ) sliding into the door bar to lock it: key -> ケイ, and the bar secures the door.