granary, storehouse
庾 centers on a building for storing grain or goods: a granary or storehouse.
庾 combines 广 (building) with 臾, which likely contributes the sound. The character originally referred to a granary or storehouse.
Under the building roof 广, the inner shape 臾 looks like a container with a lid, storing grain safely inside a granary.
For ユ, imagine you opening the granary door: you -> ユ, and you see the stored grain.