The extension, "Reddit saved posts organizer", collects the users reddit username and their feed key. Both pieces of information is gathered from this page: https://www.reddit.com/prefs/feeds. The page is only available, when the user is logged into their reddit account in the browser.

The extentsion also collects the users saved posts and comments from their JSON-feed (found on the link above).

The extension saves the username, feed key and saved posts and comments locally on the users browser. There is no sharing this information between servers, third parties or syncing to the users browser account.