Sometimes someone with a CS or SE degree, sometimes someone who learned to program as a hobby while doing something completely irrelevant like Music, English, bar tending or high school and sometimes the cowboy coders themselves with more experience. There’s an enormous amount of theory in programming which is highly relevant to many, many people but you can be amazing at CS theory and write scientific code that’s garbage, uncommented spaghetti like the Imperial epidemiology model. At the other end you can have a great grasp of how to write clean, modular, well commented code and have no idea how you would start parsing a text file to extract all nouns or some other introductory undergraduate project for one of the infinitude of topics in CS.