the forehead, to kowtow
顙 centers on the forehead as a body part, and by extension the act of touching the forehead to the ground in a deep bow or kowtow.
顙 combines 桑 (mulberry) and 頁 (head), likely as a phono-semantic compound where 桑 hints at the sound and 頁 indicates the head, together meaning forehead.
The left side 桑 looks like a mulberry tree with leaves, and the right side 頁 is a head. Imagine a mulberry leaf stuck to someone's forehead, marking the spot.
For ソウ, picture someone kowtowing so deeply that their forehead sows seeds into the ground: sow -> ソウ, and the forehead touches the earth.