button
釦 centers on a small fastener used to join fabric: a button. The meaning is concrete and specific.
釦 combines 金 (metal) with 口 (mouth), likely indicating a metal fastener with a round opening or shape. The exact historical development is uncertain.
Metal 金 with a round opening 口 like a buttonhole: 釦 is a button.
For コウ, imagine a coat button that makes a crisp 'koh' sound when fastened: koh -> コウ.