Quasars are the brightest sustained objects in the universe, the cores of distant galaxies where a supermassive black hole is actively swallowing matter and blazing as it does. That makes them easy to spot and thoroughly awkward to study, because the glare from the middle swamps the galaxy around it. It works regardless of how bright the foreground galaxy happens to be which is precisely what you want when the foreground galaxy has a quasar in it. Everett McArthur, a graduate student at Ohio State, went looking in the catalogue from the Dark Energy Spectroscopic Instrument, which holds around 800,000 quasar spectra. There were nowhere near enough real examples to learn from, so they manufactured them, blending genuine quasar spectra with genuine background galaxy spectra to produce mock lenses.