wlroots is a library that can be used to implement a compositor like KWin or mutter (GNOME). In practice wlroots is used in Sway, Hyprland, river, and more.
What wlroots-based compositors, KWin, and mutter share is that they implement a similar set of the display protocol Wayland. E.g. KWin and Sway implement the Wayland extension wlr_layer_shell