dumpling made of glutinous rice
糉 centers on a specific food: a dumpling made from glutinous rice. The meaning is narrow and concrete, without broader abstract extensions.
糉 combines 米 (rice) with 㚇, which likely contributes the sound and relates to wrapping or bundling, fitting the idea of a wrapped rice dumpling. The exact historical development is uncertain.
The left side is 米 (rice), and the right side 㚇 looks like a bundle wrapped up. Picture sticky rice being wrapped into a dumpling.
For ソウ, imagine sewing a rice dumpling shut with a quick 'so' sound as you pull the thread tight: so -> ソウ, and the dumpling is sealed.