Having tried to dig into this a while ago, they installed carbon filters.
Most of the stuff for sale as “carbon” filters has too little carbon to do much. You want quite a lot, and, for some gasses, you can get special impregnated carbon or other media.
Tangential, are there DIY methods of activated carbon? I got pellets, impregnated sheets and a packaged one for my air purifier, but I dont know when they are helping and when they are exhausted.
To tell when carbon filters are exhausted, the best way is to use an indoor VOC monitor.
Air filter AC is specially prepared, so making your own DIY charcoal (while possible) isn't nearly as effective. You can mostly regenerate/desorb the carbon by heating it to 400F. Never use your regular oven though, not least of which because it reeks. Get a cheap toaster oven and disposable foil pans and do it outside, in a location where it can't possibly catch anything on fire.
Some of the stuff on Amazon sold for indoor marijuana growing is probably legit, and you can easily buy cylindrical cartridges and attach an axial fan.
Or you can buy from one of the most reliable e-commerce sites in the US:
Most of the stuff for sale as “carbon” filters has too little carbon to do much. You want quite a lot, and, for some gasses, you can get special impregnated carbon or other media.