Whats the best way to clean concrete garage floors stained with oil?

i have used cat litter

Kitty litter or sand will sop up the excess oil. Then purchase a good degreaser from any hardware or home improvement store formulated for concrete floors.

muriatic acid is the only concrete cleaner that truly works, in my experience. Be sure to follow all instructions on the bottle. Also, it is going to corrode whatever it lands on. Wear protective equipment. Don't expect it to be good as new. It might behoove you apply some garage floor covering of some sort once it's clean.

Cover small areas with kitty litter and grind it in real good until it’s a powder, do a section at a time. I would probably soak the area good with a degreaser before then again after the kitty litter then scrub with hot soapy water..

Here are some other ideas using items you likely already have around:

https://www.lewisdrug.com/blog/five-ways-clean-garage-floor-stains?utm_source=Pinterest&utm_medium=PinterestBoostedPin9&utm_campaign=LGBlog

Just be sure to rinse well between attempts, you don't want stuff to ruin your concrete!

https://www.aol.com/diy/how-remove-oil-stains-concrete/

The muriatic acid really does work. Just be forewarned that you will need to wear a mask or something and run fans to keep the fumes from reaching your lungs or eyes. It will do a good job on concrete. It also can be used in toilet bowls if nothing gets them cleaned.