To build a snap package for Linux, first, set up the environment as per the instructions here. This includes installing snapcraft and multipass. Then, build the snap with:
snapcraft
If there are errors, you may need to delete the build directory and try building again. If that still does not work, delete the build directory and run
snapcraft clean
to remove all previously downloaded and built files, then try building again. This is basically creating a new VM so it will take time to build again.
To build for Windows, run:
flutter build windows --release
which will build the executable .exe file and other required files and put them into the build\windows\runner\Release\ directory. The files in this directory, plus the data directory below it, are required to run the application. In addition, msvcp140.dll, vcruntime140.dll, vcruntime140_1.dll from C:\Windows\System32 are also required for a standalone distribution.
To build an APK, run:
flutter build apk --release
which will create an APK in the build/app/outputs/apk/release directory.
If an Android device is connected to the computer via USB, you can install the APK by
flutter install