Unknown unknowns: The things that you don't know are possible and thus don't even know how to start solving. You will accidentally reinvent the wheel, most likely very badly. Or give up right away.
Known unknowns: An experienced developer most likely unconsciously knows (has read on HN/Reddit/Book/Blog some time in the last 20 years) that a thing they encounter has been solved already, at least partially.
This allows them to start searching for the answer with a few key words, because they know it's possible but don't know how it can be done.
Same comes with having a network of people who know different things, I know the bare minimum about professional pentesting, but I know a few people who I are absolute pros in the field.
I feel like my brain is full of "I read a headline about this once" tidbits, in part because of HN. It's like I don't know the answer, but I know the question has been solved before so I know there is an answer.
I wouldn't say that. What I feel like I've accumulated in knowledge over time is I can look at a new problem and without having the solution/encyclopedic knowledge, I can know in my gut that (a) there is a solution, and (b) the solution I'm looking at is not the solution.
Merely being able to look up things on the internet without these two things means you will either give up too early or settle on the wrong solution.
This is the most succinct way I can describe what I think >a decade of experience is worth compared to being fresh out of college.