For me a big part of it is how gestures track, the curves used by animations, inertia, bounciness, etc. Under iOS it all feels refined and naturalistic, whereas Android’s counterparts land somewhere in the uncanny valley and come off as more mechanical (fitting given its name, I suppose).
Aside from that, in general you can feel more “seams” between components and rough edges all throughout Android’s UX. It reminds me of how the Linux desktop experience used to be several years ago actually, except Android seems to have gotten stuck for unknown reasons where Linux DEs have continually improved.
