How to get rid of CAT urin in my basement?

You will need to find something that will neutralize the ammonia in the urine. There are enzyme based soaps that can work. Check out your local pet store in the cleaning section. There is also a fragrance free version of Febreeze especially formulated for pet odors, that I find that works for my dog's accidents.

Enzyme spray , found in pet stores.

Try an oxygenator machine. You leave the house and remove all plants and pets and let it run. Done.

I would scrub it, get a good odor neutralizer and seal it

If it were me, I would bring in the remediation people for a consult, even if you do the work yourself. They deal with horder houses all the time. Typically TSP or a vinegar wash ceiling to floor will work, but this might take an extra step.

I agree with using the enzyme odor killer from the pet stores. Fiirst wash the walls and floor down with TSP then use the enzyme odor cleaner. Once all is dry paint the walls and floor with Drylock waterproofer. Seals out moisture it'll seal any residual odor in.

I am actually trying to do the same thing to the house we are selling. Nothing like the scenario you are facing, but, cat litter boxes and messy cats can do a number on basement floors. I find disinfectant, a scrub brush, gloves and a wet/dry shop vac are my helpers. For such an extreme situation like that many cats, it might be healthier to look into a professional cleaning. If you do it yourself, since you are dealing with feral cats, there may be pests such as fleas, too. I wish you luck!

I would definitely try to clean them as best as possible and look into a commercial grade cleaner. Once that's all done I would seal the floors so nothings seeps through.

Thank you so much everyone. All great solutions.