[elesiuta]

If they're on the same network, cross platform, open source airdrop alternative https://github.com/localsend/localsend

[tamimio]

I like LocalSend and Landrop. The latter performed better when I sent large files. However, neither of them does it automatically; you have to manually do it every time, which is okay since they don’t claim to be sync software.

[SushiHippie]

Maybe syncthing fits your use case better?

https://syncthing.net/