You find a website that creates a link between 2 clients, and the file is shared via https (maybe the file is decrypted at the server and encrypted again using your public key). Sounds easy if both clients don’t have a public IP (to get SCP to work, at least one client needs a public IP) and every browser now has WebRTC.
BitTorrent is a fantastic way to transfer files, except for this one hitch I keep hitting.
I wanted to about 2tb of data, mostly torrents, over a wlan from my NAS, and kept getting dropped packets. Eventually I gave up on smb and ftp, and set up opentracker (a simple download and build, with iirc two config options) on my windows desktop under WSL, and swapped the tracker to a local address; after this, it worked perfectly.
I remember one customer (a state department of education) in particular had a server at each school with reasonable private bandwidth but unreliable routing and dismal internet connectivity, so we used the BitTorrent protocol for eventually-consistent asset distribution between their sites.
See also: using NNTP as your pub/sub message bus.