Software engineer? You think I cite all the code I’ve ever seen before when I reproduce it? That I even remember where it comes from?

You don't?

If you reproduce something, usually you have to check the earlier implementation for it and copy it over. This would inevitably require you to look at the license and author of said code.

Assuming of course, you're talking about nontrivial functionality, because obviously we're not talking about trivial one-liners etc.