For strictly local use, Google's Nearby share is technically FOSS but the documentation is basically non-existent and a proper Linux implementation is not here yet. Alternatives aren't hard to find though, with Mint's Warpinator or KDE Connect having worked well for me.
For non-local use (everything out of Bluetooth range), you almost have to trust a third party, unless you host yourself, and it really depends on your use case. Want to send your friend a file or host pictures of your birthday for multiple people to download? For the former magic wormhole works great, for the later you could almost spin up a nextcloud or similar (personally I like https://github.com/filebrowser/filebrowser ). Want to regularly send files from device 1 to device 2? Now sync solutions like syncthing or rsync become really viable.
If everything else fails, FTP always has your back
For non-local use (everything out of Bluetooth range), you almost have to trust a third party, unless you host yourself, and it really depends on your use case. Want to send your friend a file or host pictures of your birthday for multiple people to download? For the former magic wormhole works great, for the later you could almost spin up a nextcloud or similar (personally I like https://github.com/filebrowser/filebrowser ). Want to regularly send files from device 1 to device 2? Now sync solutions like syncthing or rsync become really viable.
If everything else fails, FTP always has your back