The IDE can generate "headers" for a new file, it's not gonna generate tens of thousands of new files by itself, let alone in a meaningful structure.

I don't think a developer would write a million line at work in their entire lifetime.

I'm not saying it's impossible to achieve. I've seen outsourcing agencies deliver 100k lines with just a few block copy/pasted many many times, but that's neither working nor maintainable software.