Year 2006AD. Amongst all the cultures and people on earth when and how did they agree to the number of the yr

Answer 1

They did not. The year is very different on the Jewish, Islamic, and Chinese calendars. But the spread of Christianity made the use common in the industrial world, and as global trade rose, its use has been globalized as the basis for most international trade, similar to the way English became the language of the Internet.

And non-Christians refer to the year as 2006 CE (Common Era). We do not use "AD" which translates "in the year of our Lord" as we do not worship Christ.