It's super easy to configure over a LAN.

You set up a folder path to sync, authorize it between two machines, and set up your configuration (mirrored sync, one way, etc).

For photos taken by my phone for example, I have two syncs set up (maybe not the best way to do it it's just how I did it in 5 min when I started and never changed it)

Phone has a 2 way sync set up with my photo folder with my desktop. Meaning, when I delete/modify a photo either at my phone or the desktop it deletes/modifies the photo on the other device.

Additionally, I have a 1-way sync of my photo folder between my desktop and my server archive, so the server only pulls in new files and never deletes or modifies existing files in the archive. So if something happens to my sync folder or if I delete something on accident then I have an archive copy, important for precious memories. Every now and then I just delete the contents of the archive folder and it immediately re-syncs the existing folder of all the stuff I actually wanted to keep. Could set up a cron job but I'm too lazy.

For Joplin I just have a folder on each device that's JUST my Joplin notes that syncs between my devices that use Joplin. Easy as pie - as soon as I turn on my laptop or desktop it syncs up with whatever changes have happened on the other two.

For syncthing I like to make a top level sync folder on each device and then nest many folders under that top level, each to sync distinct things depending on what device it is. You can just sync a main folder if you want but I like the granular control, and as you figure out more stuff to sync it scales much better if you start that way, and name folders consistently.

Just be careful in the beginning and always back stuff up. It's not TOO easy to mess up but if you did it would be unforgiving. Test it with some random stuff before you start sending anything important.