It depends on whether you are training your model on your computer (or in this case, on your Android device) or in the cloud.
If the model is in the cloud, you are correct - training it on the web means it will be available for use on your App Inventor device.
If the model is on your device, then you will need to train it both on the web (creating a model on your computer/laptop) and on your App Inventor device (creating a model on your mobile).
Kind regards
D
PS - In general, looking at the documentation at
https://machinelearningforkids.co.uk/help is the best way to get urgent answers to questions. This is a free site that I run in my spare time. While I endeavour to answer questions within 24 hours, I cannot provide immediate answers to questions at all hours.