iPhones take their time to show you everything which is received by us as smoooooth. Android on the other hand, if there are flickers or stutters - it mostly tries to get to your face as soon as possible by leaving little room for animation. That is sometimes if not mostly perceived as Android not being smooth.

This is not to say iPhones are just that. A great deal of attention and care went to build the UI and its animations. Overall it feels very natural and smooth. Pixel UI vs One UI - you can definitely see how Google eliminates stutters and the overall feel of it is smooth. One UI struggles with stutters and animation inconsistencies.