Back in the dark ages, I did do plumbing. No real license was demanded, just a functioning brain. I charged $30-50 an hour 35 years ago!
BUT, there is a catch no one seems aware of... Everyone just assumes that plumbing means water pipes, but that is not even close to what the job entails in reality. The reality is some elderly couple that have a leaking sewer pipe under their bathroom, and a huge swamp of sewage you'll be wading in and breathing while the sweat and tears roll off your face. You, your equipment and your tools will literally be covered in human waste daily. It will even find its way into your lunch, eventually.