pistil, stamen
蕊 centers on the reproductive parts of a flower: the pistil and stamen. It refers to the central botanical structures involved in pollination.
蕊 combines 艹 (grass/plant) with 惢, which likely represents clustered hearts or buds, together suggesting the flower's reproductive center. The exact historical development is uncertain.
Under the grass top 艹, the three 心 (heart) shapes clustered together look like the central parts of a flower: the pistil and stamens.
For ズイ, imagine a bee buzzing 'zui-zui' as it lands right on the pistil: zui -> ズイ, and the bee pollinates the flower's center.