Is there a tutorial place to learn about plug ins?
Not really because it highly depends on the plugin. So mostly Google is your friend.
I am looking for Syncthing in particular and I can see that there is a significant number of users are trying but I have yet to see a successful one... Is there any one from Gl.iNET technical team that manage to get it work... I mean they already added that plug ins in their devices as an option.
Everyone is trying the same thing, sync from the connected hard drive or the micro sd (no computer) to a remote computer (or server).