The assets and some? of the scripting for it are in a bundle at:

/System/Library/ExtensionKit/Extensions/WallpaperMacintoshExtension.appex

Can see the time and date being added here I think:

System6ControlPanel.program

    //
    //  System 6 Control Panel
    //  Macintosh 40th Anniversary
    //
    //  Copyright © 2024 Apple. All rights reserved.
    //
    //----------------------------------------------

    start simultaneous
        scale out
        let image = "System6ControlPanel"
        ...
        ...
        add time at 267, 165 name "Geneva12"
        add date at 274, 239 name "Geneva12"
    end simultaneous