There are several approaches to handle authentication in a React Native app, but one common method is to use JSON Web Tokens (JWT). Here’s a basic example of how you can implement JWT authentication in your React Native app:

1. User Login: When a user logs in, send their credentials to the server. If the credentials are correct, the server generates a JWT token and sends it back to the client.
2. Token Storage: Store the JWT token securely on the client-side (e.g., in AsyncStorage or SecureStore).
3. Protected Routes: For protected routes, include the JWT token in the request header. The server verifies the token and grants access if it’s valid.
4. Token Expiry: Set an expiration time for the JWT token to improve security. Users will need to reauthenticate once the token expires.

Here’s a basic implementation using AsyncStorage for token storage:

// Login Screen
const login = async (username, password) => {
  try {
    const response = await fetch('https://your-api.com/login', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
      },
      body: JSON.stringify({ username, password }),
    });

    const { token } = await response.json();
    await AsyncStorage.setItem('token', token);
  } catch (error) {
    console.error(error);
  }
};

// Protected Route
const getProtectedData = async () => {
  try {
    const token = await AsyncStorage.getItem('token');
    const response = await fetch('https://your-api.com/protected-data', {
      method: 'GET',
      headers: {
        'Authorization': `Bearer ${token}`,
        'Content-Type': 'application/json',
      },
    });

    const data = await response.json();
    return data;
  } catch (error) {
    console.error(error);
  }
};

Remember to replace 'https://your-api.com' with your actual API endpoint.