Follow the steps below to get your Plist file :-
Step 1 : Go to Firebase and Sign in to your account.
Step 2 : Click on Get Started
Step 3 : Click on Create a Project
Step 4 : Enter your project name.
Step 5: Click on Continue.
Step 6: Go to your project settings and navigate to "Cloud Messaging". Enable it to use push notifications in your app.
Step 7: Under “General Settings”, click on the iOS icon to begin the registration process.Follow the 5-step process outlined by Firebase to register your iOS app successfully.
Step 8: Now login to your Apple developer account and click on “Certificates” under certificates,IDs & profiles.
Step 9: Click on the "Keys" tab to manage your authentication key and click on the plus button to add a new key.
Step 10: Provide a name for your key and register it. Then, click on "Continue” to create the new key.
Step 11: Finally, download your key (Auth key) from Apple Developer.
Step 12: Go back to your firebase account >>Navigate to the apps section in the general settings, and select the upload button beneath the authentication key.
Step 13: Navigate to your Apple Developer account, click on "Account," and copy the Team ID from the membership details.
Step 14: Upload the downloaded authentication key and enter the Key ID and Team ID obtained from your Apple Developer account.
Step 15: In the general settings, navigate to the SDK and Configuration section. Locate your Plist file and click the download button to obtain the file.