If you are serious about developing data-driven apps in Flutter, you should study some database concepts. In particular, if you're interested in SQLite, you should have a look at https://www.sqlitetutorial.net/. Here, you'll find an extensive tutorial, with examples and use cases.
If you've never heard of SQL before, or you want to study the language itself, there's a great free guide/tutorial at https://www.w3schools.com/sql/default.asp.
The singleton pattern is a fascinating topic: should you want to investigate it further, a good starting point is the Wikipedia entry at https://en.wikipedia.org/wiki/Singleton_pattern.
