The short answer
Wallpaper Engine does not run on macOS. There is no Mac client, no beta, and no compatibility layer that makes the editor or its web-based wallpapers work on a Mac.
That does not mean the wallpapers are lost. A Wallpaper Engine wallpaper is a folder of files, and most of them describe things a Mac can render on its own: video files, images, layered compositions, particle systems and the scene's animation script. PaperFlow reads those files and renders the scene itself, so the wallpaper plays without Wallpaper Engine installed or running.
What carries over
Scene wallpapers are the interesting case. Layers, particles, puppets and SceneScript are all handled by PaperFlow's own Rust engine, drawn through Metal. A scene that animates on Windows animates on a Mac.
Video wallpapers work too, and they are the simpler path. MP4, MOV and M4V files play through AVPlayer, which uses the same hardware decoder that plays video in QuickTime.
Your own videos need no Steam and no Wallpaper Engine at all. Drop a file in and set it.
What does not
Web wallpapers are an HTML page rendered by a bundled browser engine, and application wallpapers launch a Windows executable. Neither can be reproduced natively, so both are unsupported. Anything that is not a scene or a video file will not play.
This is a deliberate limit rather than a missing feature. Rendering a foreign browser engine inside a wallpaper would mean shipping one, and it is the main reason other Mac wallpaper projects are slow.
Getting your Workshop library across
Workshop downloads go through Valve's official SteamCMD with your own Steam account, so you get the same files you already own rather than a copy of someone else's library. The app's Steam Setup page walks through installing the tool, signing in and pointing it at Wallpaper Engine.
Requirements are macOS 15 or later on Apple silicon. The build is signed ad hoc rather than notarised, so the first launch needs to be allowed in System Settings.