What are the supported target architectures?

When building your application in release mode,
Flutter apps can be compiled for armeabi-v7a (ARM 32-bit),
arm64-v8a (ARM 64-bit), and x86-64 (x86 64-bit).
Flutter does not currently support building for x86 Android
(See Issue 9253).

How do I sign the app bundle created by flutter build appbundle?

See Signing the app.

How do I build a release from within Android Studio?

In Android Studio, open the existing android/
folder under your app’s folder. Then,
select build.gradle (Module: app) in the project panel:

Next, select the build variant. Click Build > Select Build Variant
in the main menu. Select any of the variants in the Build Variants
panel (debug is the default):