put together, inner palace
冓 centers on bringing things together or constructing a framework. The meaning "inner palace" is a specific application of this idea, referring to a structured, enclosed space.
冓 is thought to depict a framework or structure, possibly two pieces of wood joined together. The exact historical development is uncertain, but the character is associated with the idea of constructing or putting together.
The top part looks like a roof or cover, and the bottom 冉 resembles a frame or support. Picture putting together a structure, like building an inner palace by assembling beams and walls.
For コウ, imagine constructing a 校舎 (こうしゃ, school building) using the framework of 冓: コウ from こうしゃ cues the reading while you put the pieces together.