Likely he has what is called a floating floor. Where the floor is poured inside the foundation. Like most basement and garage floors where the basement and garage foundation is a stem wall. My own shop had some unlevel areas, and I just took my 4' level, and added some spacers under the sleepers to make each board level, and checked across the sleepers as well. You need to put the shims every 16", so the sleepers do not bow down between the shims. I also ran a screw down into the shims so they could not slip out from under the sleepers.