I’ve had this issue with Samsung phones and tablets I owned in the past. Working as an app developer I still see this kind of problems on heavily oem-customized versions of Android.
Personally I “solved” this by switching to Pixel phones which in my experience don’t slow down even after 3-4 years of usage and updates. I believe this is true in general for phones that stay as close to AOSP/stock Android as possible.
I can’t speak about Flutter or React Native, but what I can say is DON’T use Xamarin Forms/MAUI. As a native Android developer I had to start using Xamarin after changing jobs and it’s been one of the biggest regrets of my career, honestly. Literally nothing works like you would expect it to. I understand the idea of writing the same code twice is intimidating, but trust me, nothing beats native development. Nothing. I can say with 99.9% certainty, you will regret not going with native if (or when) your app requires any vaguely complex feature to be implemented into it. Swift and Kotlin are similar enough that you can literally write the same app natively for both platforms faster than it would take you to write them in any cross platform framework (or at least Xamarin/MAUI), unless you’re making an extremely simple app with no customizations whatsoever.
I’m not sure what you mean. Flexible OLED displays have been around for a while and foldable devices are just an example of the technology in use, but we’ve had them in consumer products way before that (phones with curved edge displays, for example). The potential for flexibility has always been intrinsic to OLED displays because they don’t need a backlight. The reason our phones don’t bend and flex like the “device” in the video isn’t because of the display, but because the battery, processors, ram, speakers, ports and all other components are not flexible and won’t be for a while. The device in the video does not include those, there is a ribbon cable coming out of the bottom connecting the two screens to the actual hardware.