The administrator is above the System Manager and has all the rights and permissions for an ERPNext account.
A System Manager also has permissions to most items in the system, but the Administrator has unrestricted access.
If your ERPNext account is cloud-hosted with us (Frappe Technologies Pvt. Ltd.), then you won't be able to access your ERPNext account as an Administrator.
For cloud-hosted account, upgrades are managed from the backend. We reserve admin login credentials with us so that we can upgrade all the customer's ERPNext accounts from the backend.
Since on a single server, we may host many customer's ERPNext accounts, as a security measure, we cannot share the credentials for administrator account with any cloud-hosted user. (an exception would be if you purchase a large number of users and your account is exclusively hosted on one server).
For self-hosted on-premises accounts, the admin credentials are with the account user.