Is there an .apk for Android?

Cactus is a framework - not the app itself. If you're looking for an Android demo, you can go to

https://play.google.com/store/apps/details?id=com.rshemetsub...

Otherwise, it's easy to build any of the example apps from the repo:

cd react/example && yarn && npx expo run:android

or

cd flutter/example && flutter pub get && flutter run