to scratch
搔 centers on the action of scratching, typically with a hand or claw-like motion.
搔 combines 扌 (hand) with 蚤, which likely contributes the sound and the idea of a scratching or irritating action, similar to a flea's bite.
A hand 扌 dealing with a flea 蚤: when a flea bites, you scratch with your hand.
For ソウ, imagine scratching a sore spot: the sore feeling cues ソウ while your hand scratches.