assigning the Global Administrator
role to a user means: The user get's full access in the Azure Active Directory.
The Global Adminstrator
doesn't have any permissions in an Azure Subscription.
To grant permission in an Azure Subscription the user needs to be added in the Subscription with the approbiate role:
For Subscription:
https://learn.microsoft.com/en-us/azure/role-based-access-control/role-assignments-portal-subscription-admin
For a Resource Group:
https://learn.microsoft.com/en-us/azure/role-based-access-control/role-assignments-portal?tabs=current
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Regards
Andreas Baumgarten