The Software Updater uses a commercially available service, I’m not sure just which one is currently being used.
As you have noticed not every app that you may have on your machine will be covered by the service.
There are millions (billions) of apps out there, it would be impossible for any such service to cover everything.
As with any such service though the list of what is covered changes and apps get added all the time.
So if you have an app installed that isn’t covered currently then it may get added in future.
For example you will often see a line like this in the CCleaner version notes: