I would like to know how to use “blah blah blah,” “yada yada yada,” “so-and-so” and “whatnot.” Could you tell me the differences among them? Or, could you tell me which is OK and which is not about the examples below?

1. I went to his house, blah blah blah, I was tired.
2. I went to his house, yada yada yada, I was tired.
3. I went to his house, so-and-so, I was tired.
4. I went to his house, and whatnot, I was tired.

5. I’ve got to do blah blah blah.
6. I’ve got to do yada yada yada.
7. I’ve got to do so-and-so.
8. I’ve got to do what not.

9. Mr. Blah blah blah
10. Mr. Yada yada yada
11. Mr. So-and-so
12. Mr. Whatnot

This is a very hard concept to grasp because there are no rules! But some are more natural and there are different social acceptances. But I'll try my best to explain.

Generally, blah blah and yada yada are informal. So and so and whatnot are slightly more formal than that. If you want to be extremely formal dont use these fillers.

Blah blah and yada yada are interchangeable. You can use these when you are filling in unnecessary details.

I went to his house, played soccer and blah blah blah/yada yada yada , I'm tired now.

So-and-so is used usually for filling in hypothetical names.

The teacher said, "for examples purposes, let's say so-and-so liked playing sport."

What not is used when you use an example TOO specific and want to broaden it out.

Q: "what plants grow underground"
A: potatoes and whatnot.

I hope my explanation helped. It is a lot of information but these are the general rules and examples which we use these in.