Explanation
To enable Security defaults for contoso.com, you should first sign in to the Azure portal as a security administrator, Conditional Access administrator, or global administrator. Then, browse to Azure Active Directory > Properties and select Manage security defaults. Set the Enable security defaults toggle to Yes and select Save.
After that, you can assign Admin1 the Identity Administrator role for Au1 to enable them to manage security defaults for the tenant.
https://practical365.com/what-are-azure-ad-security-defaults-and-should-you-use-them/