You can’t change the path of folders, you need to create a new one (and delete the default folder if you don’t need it there). Te default directory setting is just what will be proposed as path when creating a new folder.
After adding devices on both sides, you need to share a folder, i.e. when/after creating the folder you need, you select your friends device under “share with” (or similar) in the folder editor.
The introducer feature is something else, see Introducer Configuration — Syncthing documentation