surplus, excess, remainder
餘 centers on having more than needed: a surplus, an excess, or a remainder left over after the main portion is accounted for.
餘 combines 飠 (food) with 余, which likely contributes the sound and the idea of excess or surplus. The character originally referred to leftover food, then broadened to any surplus.
The left side 飠 is food, and the right side 余 suggests extra. Picture a feast with so much food that there are leftovers piled high: surplus and excess.
For ヨ, imagine a yo-yo that keeps coming back up, leaving an excess of string: yo -> ヨ, and the extra string is the surplus.