Probably the two most obvious architectural differences are that a) Flutter paints every pixel on the screen, whereas React Native uses the stock iOS and Android components; and b) Flutter uses Dart which can be compiled ahead-of-time to native ARM machine code, whereas React Native code is written in JavaScript.
We think the combination of the hardware-accelerated graphics engine, native-compiled code and customizable UI widgets is a powerful one, as exemplified by demos like this one: https://www.2dimensions.com/b/225-flutter-showcase
My favorite feature of this demo is that the translucent slider control is overlaid on the animated graphics beneath: this is often difficult to do with other frameworks because they don't have the same level of control over what is painted, or can't do it with the required performance.
Your results may vary, but this demo runs smoothly on my budget last-generation Nokia 6 phone, which I bought for about $200.
Hope this helps a little?