As per Get all site collections listed in SharePoint Admin center using Rest / Search / Graph api, all site collections created in tenant are stored in list "DO_NOT_DELETE_SPLIST_TENANTADMIN_AGGREGATED_SITECOLLECTIONS".
So, you can create a simple flow like below using tenant admin account to notify user on new site creation:
Where Site Address is SP admin site URL like: https://<tenant>-admin.sharepoint.com/ & List Name is DO_NOT_DELETE_SPLIST_TENANTADMIN_ALL_SITES_AGGREGATED_SITECOLLECTIONS.
Output:
Update from comments:
I just tried creating new communication site in my tenant & received email like:
