How do I learn to handle bigger drain and sewer problems?

You can clear a simple clog, and that is a real foundation. The deeper drain and sewer problems overwhelm you because they are bigger, hidden, and higher stakes, not because you lack the ability. Every plumber who handles those started exactly where you are. The overwhelm usually comes from facing the whole problem at once. A collapsed or deep line is still just a series of steps: find it, understand it, choose the fix, do it. Broken into parts, the monster becomes a sequence you can work through. Learn the parts and the fear shrinks. Find one person who does this work well and ask to tag along or assist on the bigger jobs. Watch how they break a scary problem into steps. Skill here is built one job at a time, from simple to complex, and you are already on that road.