Such actions are audited in the Unified audit log, which you can access as detailed here: https://learn.microsoft.com/en-us/purview/audit-search?tabs=microsoft-purview-portal
In particular, you can search for the SiteCollectionAdminAdded event.
For email alerting, you can use the good old Activity alerts feature, although Microsoft is trying to deprecate it for a while now. You can still find the relevant bits in the Security portal or use the New-ActivityAlert cmdlet directly.
Or if you are exporting the Unified audit logs to external system, consider configuring alerting therein.