Through the process of a mutation - this being the means trough which Forerunners mature as they age. It involves biochemical mutation and genetic engineering which manifests as both physical and mental changes. Bornstellar underwent what is known as a brevet mutation, a far riskier process because mutations are meant to be a gradual process, but they were running out of time. The reason why the Ur-Didact provided Bornstellar with his imprint was because Bornstellar had to access the Domain, something he could not really do as a Manipular - a Forerunner who has undergone no genetic mutations, no physical engineering. Since there were no other Forerunners present (it is usually the father who provides their imprint for their child's advancement), the Ur-Didact had to perform the mutation on Bornstellar and provided him with his memories, knowledge, and wisdom. Over time, the imprint flourished in Bornstellar's mind and he essentially became the Didact. Since the Master Builder captured and seemingly killed the Ur-Didact in later events during Cryptum, Bornstellar was thought of as the last thing remaining of that particular Forerunner. This was untrue of course, as the Master Builder had actually marooned the Ur-Didact in a Burn (Flood-infested star system) where he was captured by the Gravemind and twisted into a weapon against the Forerunners, but in the decade or so that followed Bornstellar's mutation he had to take on the Didact's role and responsibility.