victory, surplus
贏 centers on gaining more than expected: achieving victory or ending up with a surplus. Both senses share the idea of coming out ahead.
贏 is a phono-semantic compound. The lower part 貝 (shell, money) suggests wealth or gain, while the upper part (a combination of 亡 and 口/月) likely contributed the sound. The exact historical development of the upper component is uncertain.
The bottom is 貝 (shell, money). The top looks like a container overflowing. Picture a pile of shells spilling over the top: you have won a surplus.
For エイ, imagine winning a surplus of 'A'-grade shells: 'A' -> エイ, and the shells are piled high inside 贏.