This project provides the xml files of the non-standard wayland protocols we use in Plasma.
They are installed to $PREFIX/share/plasma-wayland-protocols.
You can get the directory where they're installed by using
find_package(PlasmaWaylandProtocols)
Then they can be accessed using ${PLASMA_WAYLAND_PROTOCOLS_DIR}.
You can learn more about such protocol files in https://wayland.freedesktop.org/docs/html/ch04.html.