I can't, I just assume that it exists (coz there's a plist for everything in the OS), and I'd like to edit it to reflect reality.
There is often a plist for everything -- though sometimes Apple uses sqlite databases. And some services now seem to use client/service model even if everything is local. macOS is starting to resemble other OS more than traditional UNIX or even MacOS X...
What is plutil? Is that part of Xcode? Or is it for use in Terminal? And for XML editor, perhaps BBEdit?
plutil is a command-line utility for converting a binary plist into an XML plist. You can then edit the plist with a text editor (or to be safer, something that ensures compliance with XML structures). I believe it comes with the base macOS but you have to access it from Terminal (or similar). As a general rule, editing plists outside of the standard interfaces (e.g. System Preferences) requires going to the command-line or installing a 3rd party tool.
True.
Just to clarify, this is what I'd like to edit, if possible.
This article by Howard Hoakley may provide some helpful context:
Introducing a new version of SystHist which lets you see everything that Apple, the App Store, and silent updates install, for El Capitan, Sierra, and High Sierra.
eclecticlight.co
Note things have moved since the links in that article so scroll down on this may be helpful if you want that tool:
Skint – a watchful eye on security settings Updated! Checks key security settings and features including SIP, SSV, Gatekeeper, XProtect, XProtect Remediator and macOS security updates. Runs a…
eclecticlight.co
Then the list of current installed packages is maintained by the "pkgutil" I mentioned. From Terminal, "pkgutil --pkgs" to get a list of everything it knows about (System Information/Installation may show things that were installed and then officially removed since it is more of a history/log than an active list).
Stepping back, unfortunately macOS's package management is relatively weak (compared to even Windows which I don't love either). It's half-done and your situation is an example of what should be easier for anyone.
In any case, if you want to remove macOS's knowledge of an installation, you can do:
pkgutil --forget PackageName (where PackageName is something like 'com.apple.pkg.XProtectPlistConfigData_10_15.16U4385')
This however won't remove the files. It just removes the entry in the installation database. It also probably won't erase the entry from Installation log from System Information/Installations. I believe that is a running log of everything installed even if it is removed. However, I don't believe most tools use the Installation log to find say plugins or other files so it can be ignored.