Like I have created an app but I want that user just sign in and store their credentials. Then with the help of those credentials we will help them to use other features in app lying in other screens.
You can use tiny db, or start value etc
As Horizon mentioned, you can use TinyDB to store data in the local storage of the device. This data can be fetched/read/changed from any screen in the app.
You can use start value for it
Pro: Works online, stores multiple megabytes of data, and can store images and text.
Con: Working with this component, if other users access the properties of this component of your app, they can read your usernames and passwords as the data is stored in the same database.
Pro: Accessible for your app, working offline and takes a short time to retrieve data. Can store up to millions of tags without crashing.
Con: Only stores data within your phone, and if you want to create a series of apps using the same login system, TinyDB does not work as it limits the data within your app. Bad for users using multiple devices too.
Pro: A good start for you to learn online data management.
Con: Very limited, you can only store 2K tags and 500 characters for each tag.
Pro: A powerful tool; you can use both the Web component and the FirebaseDB component for this. Manage your own security rules, stores text for Realtime Database and images for Firebase Storage. Virtually unlimited.
Con: Take extra care for your "security rules"; your data is vulnerable when hackers obtain your database URL and you do not have strong and secure security rules.
You have asked this a number of times. Stay with those topics not here.