Arduino 101 MIT App Inventor extension
Neither the Uno nor the Mega have the hardware required for Bluetooth. I have no idea what “The Arduino 101 extension” is (you don’t provide a link - and you should.)

If that’s only software, it can’t possibly provide a feature that requires hardware support. You’ll need to add a Bluetooth module to your Arduino.

After doing some searching, I found this link: https://randomnerdtutorials.com/getting-started-with-mit-app-inventor-2-and-arduino/

It discusses using the MIT app inventor 2 and a hardware Bluetooth module to communicate between an Arudino and an Android device. I don’t know if they support other desktop or mobile device platforms or not.