leftovers, residue, remains
賸 centers on what is left over after the main part is taken or used: leftovers, residue, or remains.
賸 combines 貝 (shell, money) with a phonetic component that likely contributed the sound. The exact historical development of the upper part is uncertain.
The bottom 貝 (shell) suggests something valuable, while the upper part looks like a container. Imagine a shell used to hold leftovers after a meal.
For ヨウ, picture a leftover yolk (ヨウ) from an egg, sitting in a shell (貝) at the bottom of the kanji.