Basically, drain techs unclog drains with drain snakes or augers. They can also scope the drain with a 100’ inspection camera to see what is causing a clog. Could be roots ingressing a drain or a clay pipe that’s cracking. That’s where the big money is because replacing the pipe is the only long lasting solution. Some companies do pipe lining but that’s a bandaid for more year instead of decades. Replacing the pipe can involve excavating and concrete removal. Gets expensive fast. We’re doing a job that involves excavating 8’ down to the main drain, replacing 20’ of pipe and then covering it back up, demoing concrete to replace the basement line, and repairing concrete. $15k and the tech gets 8% of the sale and 10% if they do the work for a total of 18%. $2,700 gross isn’t bad for a hard day or 2 days work.