When I read code, I understand it, its intentions, and its output. I'm not simply replicating things that I've read before. I learned about inheritance in java in school. When I go to write python, I need only look up the syntax because I understand the logic.
But what matters is the output, no ?
Also, some people explicitly avoid reading codebases to avoid duplicating by mistake some copyrighted code.
Not sure if this is true but I heard that it used to be forbidden to Microsoft developers to read open source code.
One problem is that copyright is not well-defined. They just have to convince a court that their use is "transformational" and not a "derivative work" and they're home free. Then it doesn't matter what the license says, because Microsoft isn't licensing your work.
It's not clear to me how it is copyright infringement if the generated code is truly new. Of course, if some code output is very similar to existing code, then it is obviously copyrighted.
When you read code from an open source GPL repo and are inspired by it to write your own code, it's obviously not copyrighted.