I didnt found any hint in 5th Edition.
From 3rd Edition I just remember: a Magus have to challenge an Archmage, the Archmage sets a contest. If the magus win, he get the title, the old archmage loose it ... which would lead to lesser and lesser archmages in the order and to they question: how they first archmages became archmages?
How does that work in 5th Edition?