Someone from MIT will reply soon to your specific questions Ali.
MIT App Inventor 2 is a Web application. That means App Inventor requires the Internet to work both with the Companion and the emulator.
Third party developers have created a clone that resides entirely on a PC that is not MIT App Inventor but works like it. It is generally used by schools and developers with Web connection issues.
Perhaps you can use the clone. See Download App inventor offline
Regarding the MIT App Inventor
Getting Started with App Inventor
- Setup computer and phone/tablet or emulator for live testing as you build
- Preview the IDE
- Watch a beginner video tutorial to get started