What does 501s mean here?

What does the phrase “happen to + V” mean?

To "happen to" do something means that it is random or by chance. In the case of the video here, "happen to know a thing or two about scary" means that by chance she knows about scary things. You can use the expression "happen to" whenever you are expressing that something was a coincidence or was done by chance and was not known before. Ex: I happened to see my neighbor at the store today. Ex: The dinner she cooked for us happens to be my favorite.

What is "or so"?

You use the phrase "or so" as a way of saying that the number before is approximate and may actually be more or less. Ex: We will leave at nine, since the drive is an hour or so. Ex: My school raised two hundred dollars or so for charity. Ex: Matt got me a room there for a week or so when I first came here. Ex: The driver usually spends four hours or so helping to load and prepare his truck.

What does "core" mean? Could you give me some examples?

"Core" refers to the most central or important part of something. Synonyms include central, key, basic, and fundamental. Ex: My core mission is to make a positive impact on the world. Ex: The core idea of this essay is that mental health is as important as physical health.

What does 501s mean here?

501s are the trademark product of Levi's denim jeans. So, 'Bring those 501s a bit closer' is 'asking for someone (who is wearing 501s) to come closer'.

How nuance will change if "like this chemical thing" replace to "like a chemical thing"?

Both "this" and "a" actually work in this context. You can swap out "this" for "a" without changing the nuance of the sentence. Many people use "this" when describing things in conversation, but "a" also works well here. Ex: He's like this great big teddy bear. = He's like a great big teddy bear.