Today more then ever before licenses are being given away for plumbing and drainage applications.
I took a test not that long ago in one area that took me a full 20 minutes to pass with a 96% and get the local license for Master plumber in this area of.
Some localities only require paying a fee and learn as you go.
Look at some of the franchises advertising “plumbers trained in 12 weeks” This sounds like their employees are really stupid as I saw the work some of these non trainable do and I personally feel a monkey can be trained in 2 weeks to do the same caliper of job.
If you honestly took your time to learn to properly lay drainage lines and you know the codes as far as compacting, shoring and pitch and location of clean outs and which materials for which application you should honestly have no problem passing a license for “Master Plumber”
There is nothing complicated about figuring fixture units and peek demand just about anyone with a 6th grade understanding of math can pass most of these exams.
Just ask your local building department as to the local requirements
In the worst case you can find a licensed plumber willing to PIMP OUT (cover up) you under their license for a fee.
Although this should be considered criminal a lot of local inspectors and building departments over look this type of activity as the money talks and who knows what would happen if the code actually made everyone working on the job to have a Valid Master License not just a journeymans card.