https://www.youtube.com/watch?v=_CXXVFPO6f0
"Firebase Remote Config allows you to change the look-and-feel of your app, gradually roll out features, run A/B tests, and deliver customized content to certain users, all from the cloud without needing to publish a new version of your app."
With Remote Config you deploy the code, then decide what to show to users post release. This gives Apple a chance to review all the code submitted to the App Store.
In the case of rolling out features an A/B tests, you'd make a release including a feature, but only enable it x% of your users using RC. You can then obviously enable it for everyone if it passes your A/B test or if you're happy its working.