It doesn’t copy data, no. Symlink is short for symbolic link. So it’s a pointer to another location. But it might be useful for you. Taking a guess at your goal, here’s a relevant example.

Say you moved all of your emulation stuff stored under /media/largehdd/retroarch. You could then symlink that directory to ~/.config/retroarch like so:

ln -s /media/largehdd/retroarch ~/.config/retroarch

That data is still stored on the large drive but will now also show under that symlinked directory.

Yes you can, although this might be better done with rsync - and periodically runnind the syncing command.

But syncthing does basically the same thing plus you can sync between multiple devices on the same network.

I sync my laptop config with work pc this way.

Edit: typos, damn mobile