Who was the architect of the vatican?

Google. More than one. Check out the difficulties of designing and building the dome of St. Peter’s.

(Hints; check out Michaelangelo, Bramante and Bernini.)

Repeat after me. “Doing my own research is fun and useful later on.”

You mean it wasn’t God?

You don’t predict history. The history of the Vatican City and its various architects is documented and not mysterious. Here’s one timeline

The development of the Vatican is considered to have started with the first papal residence, which dates from the 5th century. Emperor Constantine I had the basilica built, on the site of Nero’s (54–68 AD) Circus and also what is believed to be the tomb of St. Peter, apostle and first Pope.

Some of the details are murky but most are straightforward.