I have a car telemetry logging android tablet without internet connection (in the car).
While I drive, the tablet connects to my smartphone wifi hotspot. Car telemetry gets synchronized via syncthing to this phone.
When I get home, carrying only my phone, it then gets synced to my home computers/servers. This way, the data magically travels from my car to my home, even though there’s no direct connection between them.
For my mobile devices, I also use syncthing as a remote file manager. I share a whitelist of the internal memory (via ignore-list), so the syncthing share only includes important stuff (camera folder, telegram/whatsapp, Downloads, etc).
I can manage the mobile devices’ content from my computer, remove blurred photos, back up important Downloads, etc.
BTW, this white-list took foreeeeeeeever to set up correctly, involving huge amounts of trial and error. And it doesn’t support all possible cases I have, since the ignore-list format cannot represent my desired white list.
If the devices’ storage gets full, I can then move stuff from the mobile device share, to folders shared only between the bigger computers. E.g. too many photos? Move 2gb of them to the larger superset folder (which currently has maybe 150gb of photos/videos). This larger folder is syncthing-shared between only the non-mobile devices.
I also use syncthing to manage a smaller selection of music that can fit the mobile devices. So I share a “portable-music” folder that will contain maybe 2gb of music, out of the bigger collection of music that the desktop/server computers hold (with syncthing folders too, between themselves only).