2 Answers
+ 2
Mobile apps, which manage their data localy, do not need an extra backend language.
They work with a localy database and in most cases with the SQLite.
https://docs.flutter.dev/cookbook/persistence/sqlite
You can find out in web how it can be done in flutter. More information about SQLite you can see here:
https://www.sqlite.org/index.html
+ 2
This question is better suited for a search engine since Flutter is not a topic you can learn currently on SL. There may be permissions that you can work with to achieve what you wants. Always be sure to check the docs.