Then how do you work with this: https://news.ycombinator.com/item?id=18442941

I did that job, just after university, but that is not my comment. I bookmarked it though because that person said it so well.

You will write bad code, because what you already find there - and that one company is not alone! - is already so bad, there is no way to do a good job on top of literally millions of escalating hacks.

And don't think that you could clean this up - not even with ten years of time is that possible. You can only rewrite from scratch. Trying to rewrite even a tiny part is like picking up one spaghetti and always ending up with the whole bowl on your fork.