rice dumplings steamed in bamboo leaves
粽 centers on a specific food: rice dumplings wrapped and steamed in bamboo leaves. The meaning is concrete and narrow, tied to this traditional preparation.
粽 combines 米 (rice) with 宗, which likely contributes the sound ソウ. The character is used specifically for rice dumplings wrapped in bamboo leaves.
The left side is 米 (rice), and the right side 宗 can be thought of as a wrapping or bundle. Picture rice being bundled in bamboo leaves to make a dumpling.
For ソウ, imagine sewing a bamboo leaf pouch for the rice: sew -> ソウ, and the rice dumpling is wrapped tight.
rice cake wrapped in bamboo leaves