how does yuumi heal work

I have the same problem and I dont know

There's a linear scaling based on target's missing health and topped at 80% missing health.

Base value goes up to 130% additional healing at the 80% threshold. Scaling goes up to 300% additional ratio at the same point.

For example, at rank 5, base healing is 70 + 10% AP, and goes up to 161 + 40% AP on a target with less than 20% remaining HP.

https://leagueoflegends.fandom.com/wiki/Yuumi

In short, it scales higher as the target's health gets lower.