Search notes:

Registry: HKEY_CLASSES_ROOT

The keys under HKCR store a variety of subkey types, such as file name extension associations and COM registration information. Most of the subkeys are ProgIds.
HKCR merges values that are stored under HKEY_CURRENT_USER\Software\Classes and HKEY_LOCAL_MACHINE\Software\Classes.
Generally, HKEY_CLASSES_ROOT is intended to be read from but not written to.
Microsoft documentation says:
The HKEY_LOCAL_MACHINE\Software\Classes key corresponds to the HKEY_CLASSES_ROOT key, which was retained for compatibility with earlier versions of COM.

Some sub keys

Some sub keys that are found below HKEY_CLASSES_ROOT include:
*
AppID
Applications
CLSID Inventory of COM class objects.
DeviceDisplayObject
Directory
Installer
Interface
Local-Settings
MIME
Various ProgIds User friendly identifiers for CLSIDs
SystemFileAssociations
TypeLib Information about type libraries.
WOW6432Node
.ext Characteristics of files with a given file extension
Microsoft_ACE_OLEDB_12_0

No default Powershell drive

PowerShell does not define a default drive for HKEY_CLASSES_ROOT (it only defines HKCU: and HKLM:).
In order to access HKEY_CLASSES_ROOT, a drive can be created with the new-psDrive cmdlet: 
PS C:\> new-psDrive -name HKCR -psProvider registry -root HKEY_CLASSES_ROOT

See also

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts

Index

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