Read-only access to your Google calendar list and events (calendar.calendarlist.readonly, calendar.events.readonly), plus your email address to identify your account. We can’t change your calendar.
Your settings; upcoming events for the next day or so (title, time, location, meeting link, calendar name and colour); which reminders were sent; the devices you turned reminders on for; your time zone. Meeting descriptions and attendees are never stored. Your Google refresh token is stored encrypted.
Only to send your reminders. Nothing is sold or shared, and there is no analytics or advertising tracking.
Settings → Disconnect and delete my data revokes Google access and deletes everything at once. You can also revoke access at myaccount.google.com/permissions.