kiln, coal mine pit
窑 centers on an enclosed space for heating or extraction: a kiln for firing pottery or a pit for mining coal.
窑 combines 穴 (hole, cave) with 缶 (pot, jar), suggesting a cave-like structure for firing pottery. The exact historical development is uncertain, but the component combination fits the kiln meaning.
Picture a cave-like hole (穴) with a pot or jar (缶) inside being fired: that's a kiln. The same enclosed space can be a coal mine pit.
For ヨウ, imagine a kiln glowing with heat, and you yell 'Yo!' when you see the finished pottery: Yo -> ヨウ.