Unfortunately for implementing Picture in Picture is how to incorporate ads for non premium users.
Google cannot make PiP an YouTube premium feature on iOS as main feature. This would actually violate App Store guideline of having all of iOS’s API used feature available to everyone.
Google uses iOS’s media playback API, not Google’s own they developed for Android. For example, background audio.
For non-premium users, video playback is paused, just as if you leave the app to do another thing. For premium users, audio track is actually separate from video, hence the different playback behavior and allows audio to continue playing in background.
So, while Google is ready to roll out PiP, they have to figure out how to incorporate video ads so it can be released to everyone and not run afoul with App Store Rules.