COSMIC is a desktop environment developed in the Rust programming language, using the iced cross platform GUI library for Rust, and Smithay as building blocks for its compositor, cosmic-comp. Cosmic-comp is comparable to smithay's own anvil compositor demonstration, just like the Wayland project uses Weston as demo compositor.
Until the independent COSMIC environment is finished, Pop!_OS still uses its legacy COSMIC desktop, which is a heavily customized GNOME, but is not packaged for Arch.
COSMIC's first release is called Epoch, and can be installed via AUR. It is compiling all the currently available parts stitched together by System76. The easiest is to start it through a display manager, where it will show up as an additional option, besides e.g. GNOME, KDE, etc.
COSMIC comprises a compositor, library, and applets, which may be installed as parts. As of 2023-05,AUR, AUR, AUR, AUR, AUR and the editor, AUR.