small door or window, hole in the wall, to cut through a wall, dig or bore hole
窬 centers on making or using a small opening in a wall: a tiny door, a window, a hole, or the act of cutting or boring through a wall to create one.
窬 combines 穴 (hole, cavity) with 俞, which likely provides the sound. The character expresses the idea of a small opening or the act of boring through a wall.
The top 穴 is a hole or cavity. The bottom 俞 looks like a tool being pushed through. Imagine using a tool to cut a small door or window through a wall, leaving a hole.
For ユ, picture you cutting a small hole in the wall: you -> ユ, and your action creates the opening.