Rsync is whole software , not 1 file to send someone. Syncthing is good for among your PCs, again not sending someone 1 file
What do you mean with "whole software"? You can and should use rsync to send just 1 file, the command syntax is exactly the same. rsync is a drop-in replacement for scp. scp is just objectively worse in every way.
Oh ok thanks. I'm happy you spoke up. I thought rsync was just like timeshift for whole disc backup