Do not disturb and the silence switch

While trying to learn how to work with Apple’s Focus preference, I’ve met one of my many confusions about my Apple devices and software; what does Do Not Disturb mean, control and not control?

I’d like to fully understand what the silent switch on a iPhone does. I’m on a XS running iOS 15.4 if that matters.

And I’d like to fully understand what Do Not Disturb controls in Mac OS and iOS.

Thanks for any referrals to Apple sites or answers. I appreciate.

The most succinct and complete reference I’ve seen: