Bookmark collections are initially saved in a folder named "Fonts", but it can be moved anywhere else. You can also view or edit the collections from anywhere.
To add or remove a specific font from a collection, visit the Google font's page such as https://fonts.google.com/specimen/Roboto and use the extension menu.
You can also import all the fonts from one collection to another or remove them from an existing collection if you open the extension menu on the collection preview page on Google Fonts website.
This extension doesn't collect any data and the code is open source on GitHub.