How can I learn to debug real-world code and understand how its parts interact?

Use simple debugging tools aggressively. Add temporary print statements around important steps so you can see what values variables have and how they change through loops and function calls. Writing tests helps too, because a failing test gives you a specific situation to investigate instead of requiring you to understand the whole program at once.

A lot depends on the kind of program you’re debugging. Finding why a number is wrong in a small script is very different from tracking down a visual bug in a large game. Start with smaller projects and learn to narrow the problem down. Scope is especially important: some variables only exist inside one function, while others are shared more broadly. Understanding where data is created, changed, and used will make the interactions much easier to follow.

Think of debugging as keeping a growing system organized. A tiny program is easy to inspect, but once it has many parts passing data around, you need a process: reproduce the bug, isolate the smallest failing case, inspect the relevant values, and change one thing at a time. You don’t need to understand every line before beginning; follow the data and execution path that lead to the incorrect result.

Try finding small, unfinished projects and investigating their bugs instead of always starting from a blank file. Look for student repositories with failing automated tests, open issues, or broken builds. Read the code, reproduce the problem, and then check the commit history to see how someone fixed it. That’s a good way to see how issues arise when data moves between functions or changes shape inside nested loops.