Which tenants in the O365 management portal can be operated by users from multiple tenants?

Question

There is an environment where there are multiple tenants in the directory.
In your Azure account (Main User?) An administrator account is a recognition that automatically belongs to each tenant.
If you log in to the Office345 Manage portal with an administrator account in this environment, you will only be able to work with the prescribed tenant.

I want to enable AZURE AD PREMIUM P2 for use with non-default tenants, but I can't operate the tenant.

Do users who belong only to the desired tenant need to log in to the Office345 Manage portal?

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In the following example, you want to assign AZURE AD PREMIUM P2 to "my-ad".
I'm sorry for my lack of English. Thank you for reading.

e.g.

AzureAccountUser: 
 foo@example.com

AzureAD:
  tenant: default
    users:
      admin@fooexamplecom.onmicrosoft.com (foo@example.com)
        role: global administrator
      user1@fooexamplecom.onmicrosoft.com
        role: global administrator
  tenant: my-ad (original domain: foo.example.com)
    users:
      admin@fooexamplecom.onmicrosoft.com (foo@example.com)
        role: global administrator
      user2@foo.example.com
        role: global administrator

Answer 1

The O365/M365 admin center does not allow you to manage any additional Azure AD directories. You can only manage your own, or if you are a partner, manage tenants for which you are "partner on record". Additional scenarios will be supported in the future, as detailed here: https://techcommunity.microsoft.com/t5/microsoft-365-blog/new-organization-switcher-in-the-microsoft-365-admin-center/ba-p/1165543