Both the rendezvous/mailbox and transport servers are available under an MIT license, though not every client makes it easy to use your own rendezvous.
I personally use the rymdport GUI client and the rust CLI.
I wrote my own. I aimed for a different UX than most services. For my use case I have a few devices that I often share files between. So opening the tool on both devices was a bit annoying. Instead you select the file on the first device and you get a push notification on the other. Then the transfer is done over WebRTC (locally if possible). All communication is done end-to-end encrypted and over your browser's push service.
@7_Stipend_Jackal Not really sure what you mean by "like Localshare". Is that a specific piece of software, or do you just mean sharing files between two devices on a local network via whatever protocol?
I've played around with croc a while back just to test, seems okay.
Depends on where you want to transfer your files. Do you want to sync your files to a server? Syncthing is perfect for that. SFTP and SCP also work well enough for that. Do you need to send a file to somebody else? Check out croc or portal.
The easiest thing would be to mount a remote network storage as a local directory. This way you can easily access it to read and write through normal software.
"File Browser" or "Sharry" allow you to make public links to share files with others BUT you need to run them on a server that is accessible to both parties.