For several years, Apple added things to the GM release that was not in any of the betas. The premise was that they wanted to keep some things secret and "one more thing" the user base at release time - but that stuff NEVER got tested the way other stuff did because they were not in the betas.
I did not run any of the iOS15 betas this year, so I cannot say how the CarPlay garbage worked in beta... but it's an absolute cluster**** now that it's live in production... I still cannot manage iHeartradio, SiriusXM and Downcast via the CarPlay interface... so I am not a happy camper.