This post (Windows Update Agents Vs WMIC QFE GET) from @RRUZ should get you started:
WMIC QFE GET command is equivalent to execute a query with the
Win32_QuickFixEngineering WMI class, since with Windows Vista, this
class returns only the updates supplied by Component Based Servicing,
in olders windows versions uses the
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Hotfix
and HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Updates registry keys to
enumerate the updates.
The IUpdateSearcher interface which is part of the Windows Update
Agent API (WUA), can be used to enumerate all the fixes including the
installed via the CBS, Updates supplied by Microsoft Windows Installer
(MSI) or the Windows update site, and so on.
And to get a deeper dive, check this article search for installed windows updates using Delphi, WMI and WUA shown in this post Windows API equivalent to "WMIC QFE Get" by @RRUZ:
WMI (Windows Management Instrumentation) using the
Win32_QuickFixEngineering class, you can retrieve a small system-wide
update, commonly referred to as a quick-fix engineering (QFE) update.
Starting with Windows Vista, the Win32_QuickFixEngineering class
returns only the updates supplied by Component Based Servicing (CBS),
so some updates are not listed.
WUA (Windows Update Agent) using the Windows Update Agent API is a
best option to retrieve the list of updates, you can access the
interfaces and objects from this API from delphi importing the
wuapi.dll file or creating a late-binding com object using the
Microsoft.Update.Session GUID. the next samples uses the late-binding
way.
Additionally for people searching for the way to get all installed updates in the system, which would work for a very wide range of Windows Server Versions (WS 2003 to WS2022 - PS 1.0 or higher) you will need to query both QFE and WUA updates. This might get you duplicate updates so you just got to parse those out.
