Posted by: CTN 0452 9
A "smart AI" is created by sending signals through a brain's neural pathways. Unfortunately this is fatal, so they would usually just use dead people. Since Cortana was needed to be the best they could produce, the UNSC flash cloned Halsey's brain and used the clone to create Cortana.
And eventually she was combined with forerunner data to make the blue chick we all know and love.