What a tenant and provider administration mean
A tenant is your organisation's Microsoft cloud environment, containing users, services and settings. A provider may administer it through its own portal or hold an administrator account on your behalf.
Administrator access, the reseller relationship and the licence subscriptions are different things. Being unable to use the full Microsoft admin centre is a reason to investigate access. It is not proof of legal ownership or evidence that every mailbox must be migrated.
For a business that wants more control, the objective is clear administrator and recovery access, documented responsibilities and a suitable licence supply arrangement.