For a detailed writeup on using JSON with Flutter,
check out the JSON tutorial.

Can I build 3D (OpenGL) apps with Flutter?

Today we don’t support for 3D via OpenGL ES or similar.
We have long-term plans to expose an optimized 3D API,
but right now we’re focused on 2D.

Why is my APK or IPA so big?

Usually, assets including images, sound files, fonts, etc,
are the bulk of an APK or IPA. Various tools in the
Android and iOS ecosystems can help you understand
what’s inside of your APK or IPA.

Also, be sure to create a release build
of your APK or IPA with the Flutter tools.
A release build is usually much smaller
than a debug build.