Generally you'd put down some form of litter/substrate in both coop and run to absorb poop. Some folks do leave the run bare but it can make it harder to clean up, and doesn't control mud or stabilize the soil, or manage odors.
Most popular options are sand and deep litter method:
https://www.backyardchickens.com/threads/deep-litter-method.70/ - the appropriate choice would depend on things like your poop management style, available materials, drainage/dampness issues, etc.
I use deep litter because I get a lot of rain and my run is unroofed, so it helps stabilize the soil and provides drainage. I harvest wood chips off from tree branches, along with dried leaves, dried grass, garden and weed trimmings. I used to have a massive mud problem to the point that the thick mud was pulling the boots off my feet. Now there's no mud or bad odor even after lots of rain.
These chicks have no issue walking on the run floor after a heavy night of rain. The grass outside had large puddles, but not in the run.
View attachment 2460585