You can add Admin1 as a co-administrator to the Sub1 subscription.
You cannot add Admin1 as a co-administrator to the RG1 resource group, MG1 management group, or VM1 virtual machine.
Co-administrators have full access to all resources in a subscription, including the ability to create, read, update, and delete resources.
To add Admin1 as a co-administrator to Sub1:
In the Azure portal, navigate to Sub1.
Click Access control (IAM).
Click Assign role.
Select the Co-Administrator role.
Select Admin1 in the Select drop-down list.
Click Assign.
Once the role has been assigned, Admin1 will have full access to all resources in Sub1.
Note: Co-administrators can only be assigned at the subscription scope. You cannot assign co- administrators to resource groups, management groups, or virtual machines.