Kernel extensions, system extensions, app/Finder/Safari extensions are all very different, as explained here.
Finder Extension
Searching for third party Quick Actions, I came across the excellent Yoink, the only app which seems to have solved them. Or has it?
The built-in Quick Actions and those bundled in installed apps are managed by the daemon pkd, and PlugInKit. Not so Automator workflows, though.
The problem of Undo is a major consideration when building or using any Quick Action, Automator workflow, AppleScript, or other lightweight solution.
Making your own Quick Actions in Automator is very simple, provided that it has the actions built in or provided with apps which can do the task.
One of the significant features of Mojave, Finder Actions deliver significant features without you having to open an app.Are they extensions, and where are they installed?
The Eclectic Light Company 