There are two scenarios:
Organisations are created by an Account Owner
During the creation of an Organisation by the Account Owner, all existing Account Managers are automatically added to it as Administrators.
Any Account Managers added after the Organisation's creation are also automatically added as Administrators for all Organisations linked to this business account.
Organisations are created by an Account Manager:
When an Account Manager creates an Organisation, the Account Owner is automatically added as the Account Owner (and an Administrator) of the new Organisation, and cannot be removed.
When an Account Manager creates an Organisation, all existing Account Managers are also automatically assigned to the Administrator role.