Windows Installer reference counting implementation on the registry

The old reference counting strategy was using the SharedDlls key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\SharedDlls

This is legacy, for many good reasons.
But where is the new reference counting strategy implemented?
The "new", in quote because it has been around for a while nowadays, Windows Installer way is implemented under a different registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData

where the key S-1-5-18 is for per applications installed machine-wide, while the eventual other SIDs are for the user applications.

Where is Orca?

You have got the Windows SDK (in my case Windows 7 and .NET Framework 3.5 SP1), and you are looking for Orca, where is that?

Mount the ISO (in my case the x64 version, GRMSDKX_EN_DVD.iso), or explore that with some tool (in my case 7zip x64 9.20, which you will have to use in every chance), drill to Setup\WinSDKTools_amd64\cab1.cab, you will see a WinSDK_Orca_Msi_5E20C107_DAA3_4D49_AFAE_7FB2594F0CDC_amd64 file, extract that, rename it to orca.msi, and that's it.