Right now OneNote+ only checks your own OneDrive and your recent list. Turn on Find more notebooks to also check your Teams (class notebooks) and shared sites.
Your notes always live in Microsoft OneNote. Favourites, recents and subjects are saved to a small file in your OneDrive app folder (Apps/OneNote+), so they follow you across devices.
OneNote+ is a personal client for Microsoft OneNote using Microsoft Graph. It is not made by Microsoft. Sign-in tokens are encrypted and stored only in secure, httpOnly cookies.