How to transfer files between garmin app and mobile

Hello,

I m a total newbie in developing garmin app. I have been developing apps for mobile and desktop for years but garmin is quite new to me.

I have an app working on android and Apple watch and phone, I m now on the process of adding support for garmin watch... 

I need to record activity on the watch and then send it to the companion mobile app to be post processed. For that I glhavr created a connect iq app which log the activity on FIT format  but then I m quick locked on how to transfert this file to mobile app..

I have searched for file transfer api to send the file but haven't found anything... Alors thought of reading back the file to use the toybox communication module to use standard communication for transfert but here I cannot find anything to read a local file...

Am I missing something ? Is there another way to implement such a transfer ?

Thanks I'm advance for your help

Best regards

Loic