I know you didn't mean that Emperor Constantine literally built St. Peter’s Basilica of Rome in your second paragraph but why did he order the building of the Basilica?

When did Bernini create the baldacchino?
 Your closing statement included a reference to the excavation below. What excavation and why is this important?
It would help me to see some images of the Basilica if you have them available. 
Thank you for sharing this information.