Flutter is a general purpose (and open source) platform using Dart, a general purpose language (although it was originally intended by Google as a replacement for Javascript), so I don't really understand why someone would say it's just a bunch of templates. I hand code all of my Flutter food app in Visual Studio Code (good editor support for Flutter/Dart) - the app by the way is on itch.io and I put the Flutter project on Github if you want to look at it.
I also used Flutter earlier this year on a contract to develop a telemedicine app. Flutter uses a declarative react-style way of building UIs (Flutter is generally considered an alternative to React Native) - SwiftUI is similar and I still use Swift for the iOS version of my app, but if I was starting from scratch I'd probably just use Flutter and take advantage of the cross platform support (although if you want to use different platform-appropriate widget sets then there's some extra coding).
There's plenty of doc on the Flutter website, but it takes time to wade through. I think there are some examples of 2D games, but not much 3D support, so I wouldn't use it for graphics-intensive games, and the desktop capability (mac, windows, linux) is not as mature as for mobile (iOS and Android), but it also has web support, which I think you have to pay for with Felgo.