millet, rice cakes
粢 centers on grains used for food offerings: millet and the rice cakes made from it. Both meanings connect to ritual grain preparation.
粢 combines 次 (which may contribute sound) with 米 (rice/grain), suggesting a grain used in ritual contexts. The exact historical development is uncertain.
次 (next) sits on top of 米 (rice), like the next step after harvesting rice: turning it into millet cakes for an offering.
For シ, imagine she (シ) carefully preparing millet rice cakes for a ceremony: she -> シ.