The first time you launch Twoday, it will ask for permission to access your calendar. Click on Allow Full Access. Twoday is read-only, so it will never write anything back to your calendar. There is no risk of accidentally moving, editing, or deleting an event. (This is actually a feature!) After granting access, then use Settings to select your calendars.

Yes. In the Twoday → Settings menu, you can select which calendars to show. It is placed in Settings so it is less likely to be changed by accident.

No. You continue to use Apple Calendar to create and edit events. Twoday simply presents today and tomorrow in a cleaner, easier-to-read view.

Yes, indirectly. Twoday does not connect to Google Calendar or Outlook on its own, but Apple Calendar can subscribe to those calendars. Once they appear in Apple Calendar, Twoday can display them.