android How to do so that the value of a variable can be used in all my application?

I'll give you some possibilities:

1. Pass the information by the intents of the activities

       Intent intent = new Intent(this, MiActivity.class); 
       intent.putExtra("email", miEmail);

       startActivity(intent);

       //MiActivity
       email = getIntent().getExtras().getString("email");

2. Use SharedPreferences:

Save data after login:

SharedPreferences prefs = getSharedPreferences("shared_login_data",   Context.MODE_PRIVATE);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("email", "[email protected]");
editor.commit();

Retrieve data when changing activity or fragment:

SharedPreferences prefs = getSharedPreferences("shared_login_data",   Context.MODE_PRIVATE);
String email = prefs.getString("email", ""); // prefs.getString("nombre del campo" , "valor por defecto")

There is a way with SharedPreferences with the which you can create key and values and be able to access them in your Activity

When your user signs in, you can define

public static final String MyPREFERENCES = "MisPreferences" ;
public static final String userName = "nombreUsuario";
public static final String userPassword = "passwordUsuario";

String username  = userName.getText().toString();
String password  = passwordUser.getText().toString();

SharedPreferences.Editor editor = sharedpreferences.edit();

editor.putString(userName , username);
editor.putString(userPassword , password);
editor.commit();
//redireccionas por ejemplo a otra Activity
perfil = new Intent(MainActivity.this,second_main.class);
startActivity(in);

And to access the data already saved in another Activity

SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE);
String username = getResources().getString("nombreUsuario");

  

A SharedPreferences object has an associated file containing key-value pairs and provides simple methods for reading and writing such pairs. Each SharedPreferences file is managed by the framework and can be private or shared.