I believe ProMotion on the iPad Pro is mostly felt when you use the Apple Pencil. It automatically adjust depending on the content on the screen as you won't need high refresh rate seeing static text of 30fps videos.
The reason you may not feel much difference with the iPhone is because iOS UI is already smooth. One thing Apple did really well on iOS is that the animations and movements in general flows smoothly. And Apple really focused a lot on GPU and metal framework, allowing optimized accelerations for UI draws.
This is not the case on Android where even basic app drawer opening can be jittery as many OEMs are not optimizing their skins well. I have a Poco X3 NFC that has 120Hz refresh rate. Opening the app drawer always lags/jittery simply because the lack of optimization. High refresh rate can somewhat mask that, but not much. My iPhone 7+ feels smoother all the time compared to my 120Hz Poco. Imo Android is not solving things the right way. Instead of focusing in optimizing their OS/skin/GPU driver stack for smoother UI, they just tack on high refresh rate hoping it will "fix" their problem. The high refresh rate becomes a new megapixel/PPI number race, higher is better on paper.
On a decently optimized skin like Samsung OneUI on my S21, I actually ended up disabling the high refresh rate, keeping it at 60Hz. Setting the animations to 0.5x on developer options make the phone feels faster better than the high refresh rate.