How to draw a calendar?

Question

Android

In the past, I only worked with WEB programming at the level of Joomla improvement and a couple of self-written scripts.
I would like to make a to-do list with a calendar view.
I would like to do it on Android Studio (I'm new to it) Java
And make it desktop on Java
1. How to work with data if the application is about a to-do list and a calendar, time, etc.? Maybe some pattern, for example, and the principles of storage in the database
2. How to draw a calendar? With the ability to scroll and everything you need. I mean my calendar (in the future, synchronization with Google Calendar, but not yet necessary)
3. What to look at if I need a desktop / web application in the future (I haven’t decided yet, but more for WEB because I had experience with PHP)
Suitable manual in english
All existing known analogues do not suit me as a user.
In fact, there is a lot missing in the MyPoli application - I want to make my own.