For the offline transfer of activity logs and for transferring course/media content between devices, the has been developed to use bluetooth for this data transfer.
However the course and media files especially can get quite large so transferring over bluetooth can take a long time, so an alternative option could be to use wifi direct (https://en.wikipedia.org/wiki/Wi-Fi_Direct), either instead of bluetooth or as another option.
Would be interested to hear any thoughts on this, especially whether is might be best to replace the bluetooth option with Wifi Direct or not.
Commcare uses wifi direct to transfer data files offline. Could be interesting to check out how they configured their code. (https://github.com/dimagi/commcare-android).
LMH used Bluetooth for offline transfer as in 2015, wifi direct wasn’t always found on low-end Android phones. However, this is no longer the case, and wifi direct is commonly available.
I was looking through my old notes for development of offline data transfer mechanisms for ODK-X. Their developers recommended against switching from Bluetooth to Wifi direct. They felt that Bluetooth would be easier to develop, be more stable, and have a simpler UX.
We never got far in these developments, but let me know if you want me to put you in touch with the developer.