The full process for selecting and approving Leaders is not public information. Candidates are nominated, vetted, interviewed, and offered the position if all the stars line up. Other Leaders and the WikiTree Team are involved. Team member Abby Glann, Leader Liaison, has the final word and is the contact person for Leaders and Candidates.
There is not really any hierarchy as we all play parts in this venture. We are all Wiki-Genealogists working together.
There are some things on WikiTree that can only be done by people who have been approved to do so. Only those who have qualified can work on Pre-1700 or Pre-1500 profiles for example, and only those who have signed the "honor code" can edit profiles.
Leaders have other privileges such as marking or un-marking profiles as Project Protected, adopting profiles that otherwise are not adoptable, and editing G2G posts for content. There are more. None of these "special powers" are for the benefit of the Leader themselves, but rather are there to enable the Leader to assist other members.
Education is provided in a couple of ways. There are detailed "private" Help Pages specifically for Leaders. We are in close communication with each other through google-groups and collaborate daily, discussing all things WikiTree. New Leaders are often paired with experienced Leaders who can guide them. When possible, Leaders attend the Global Family Reunion, RootsTech, or other educational group events.
The WikiTree Team are who you might consider the top-level if there is one. They have the most access and control of the WikiTree system. For privacy reasons, they are the only ones who can access every corner of WikiTree. They make sure the website works properly, seek to improve functionality, protect us from hackers, market the WikiTree name, etc... the big stuff. All to support the average person who uses WikiTee.
So maybe there are 7 basic permission levels on WikiTree right now: Guest, Genealogist, Pre-1700 Certified, Pre-1500 Certified, Leader, Team.
Maybe there are 3 groups... Genealogist work on the tree, Leaders are Genealogists who support and guide the community, and the Team manages the website and general direction.
I hope this answers your question appropriately :)