What is a variable?
A variable is a way of storing information and data in memory. As it is in memory, the data is not retained when the application is closed. If you need the data to be retained when the application closes, you should save the data in a file or use firebase cloud firestore.
If you create a variable in one screen, it is not accessible in other screens unless you define it there as well. Also, if you create a variable in a button tap event, it is only accessible in that event. To make a variable accessible in all events in that screen, don't define the variable in a event.