Here are 3 ways you can integrate your app with your user's existing calendars and how to create user calendars in your Bubble app.
Google Calendar API
The Google Calendar API allows you to read and write events to a user's Google Calendar.
iCloud Calendar API is just as simple right? Wrong!
Doing the same thing with iCloud Calendar is much trickier as Apple doesn't provide an official calendar API. You'll need to use a service like Nylas to read and write events to an iCloud Calendar in Bubble.
Lastly, the AddEvent API allows you to provide each of your users with their own calendar feed from your application.