It's almost like they use a dedicated focus/beta testing group or something, instead of making all their users join in.

I can't think of an Apple engineer who would make such a definitive statement on behalf of the company, in public. I also can't think of anyone working there who would have definitive knowledge across all apps and software, because disclosure would prevent such knowledge.

Apple introduced A/B testing to Testflight in 2017 and more A/B testing to the App store this year.

Source: I worked at Apple, but not in software.

Yep- it does this in betas, but not in production apps.