Localsend? https://localsend.org/

Probably SCP, but modern firewalls and routers automatically block that port and no one thought of making a half decent GUI

P2P within LAN?

For anything outside your LAN, 2 nodes must need a negotiator(over internet) to know each other unless the nodes already have public dedicated IPs in which case they can talk to each other securely without the need of a negotiator.

rsync

One could create it using zeroconf but i'v yet to see one software do that.