Explanation:

Explanation:
They are all Global admins so they can all modify user permission. i.e add self as owner etc.
You can be GA in one of the subscription, it doesn't mean that you can create the resources in all subscription.
As a Global Administrator in Azure Active Directory (Azure AD), you might not have access to all subscriptions and management groups in your directory. Azure AD and Azure resources are secured independently from one another. That is, Azure AD role assignments do not grant access to Azure resources, and Azure role assignments do not grant access to Azure AD.
However, if you are a Global Administrator in Azure AD, you can assign yourself access to all Azure subscriptions and management groups in your directory Reference:
https://docs.microsoft.com/en-gb/azure/role-based-access-control/elevate-access-global-admin