how are lawn care chemical companies able to get grass to shoot up and look lush even on rocky, barren and uneven soil?

i have seen very rock, uneven, clayish soil that i thought no way that would have a nice lush lawn without adding compost or topsoil to it, but I have seen "True Green" company come in and have grass growing like crazy on it and have it looking like it was sodded ...how? do they change the ph of the soil? add iron? or is it really just nitrogen fertilizer? what kind? because i have put that on my lawn and , yes it made it grow fast and green, but this is remarkable