go
徂 centers on the act of going or moving forward. It is a rare character whose core idea is simply 'to go'.
徂 combines 彳 (the go radical, suggesting movement) with 且, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is associated with going or progressing.
The left side 彳 is a step, and the right side 且 looks like a shelf or a marker ahead. Picture stepping toward a marker: you go forward.
For ソ, imagine going to sew a path forward: sew -> ソ, and each stitch moves you along the way.