ios 15.2.1 & 15.3 Upgrade

Who is responsible for the ios 15.2.1 issues my iphone 6s plus developed after last week's upgrade from ios 14.8? Audio only lasts 3 secs then cuts out for all apps then my battery goes from 100% - 1%, then 84% - 50%, then 1%, switches off, then back to 68%! The phone has been rendered useless if not plugged into a power supply while in use.

What am I to do now as I cannot revert to phone to its previous firmware?