No one is given default access to the root management group. Azure AD Global Administrators are the only users that can elevate themselves to gain access. Once they have access to the root management group, the global administrators can assign any Azure role to other users to manage it.
Reference:
https://docs.microsoft.com/en-us/azure/governance/management-groups/overview#important-facts-about-the-root-management-group
https://docs.microsoft.com/en-us/azure/governance/management-groups/overview