Doesn’t have to be SQL. But most of the time that quote refers to a relational database.
Nowadays there are graphical tools that are alright, such that you don’t have to learn a query language. For example (1), (2) or more commercial (3).
But what’s still important is doing good relational database design. Learning to look at the world as entities and relationships between them. Constraints, keys, indices. There’s books and courses on that. While you’re at that, you’ll probably learn SQL along the way, as it’s so convenient.






I think most information is passed through the global discovery api: https://docs.syncthing.net/specs/globaldisco-v3.html
STUN is only used to figure out public IPs and port and should store no information.