The docs state:
To integrate Apple Sign-In on your iOS application, you will need to install a 3rd party library to authenticate with Apple. Once authentication is successful, a Firebase credential can then be used to sign the user into Firebase with their Apple account

1. What 3rd party service are we supposed to use. Are you referring to another package on pub.dev?
2. How do we sign into

Google suggests enabling gzip compression before transferring files over the network:

In order to receive a gzip-encoded response you must do two things: Set an Accept-Encoding header, and modify your user agent to contain the string gzip. Here is an example of properly formed HTTP headers for enabling gzip compression:

Acc