Old lessons rehashed

1. Why is that the words like "American" and "Chinese" seem to be presented/written differently when following different personal pronouns? Maybe I am seeing enough of them but I see the noun or the adjective without the article for the first-person and second-person pronouns, I and You, and seeing the noun or the adjective with the acticle for the third-person pronouns.

I am Chinese. You are Chinese.

He is a Chinese. She is a Chinese.

2. When you are mentioning food items, what criteria/factors should you apply/consider when need to decide to put too much or too many?

I ate too much??? beef steak. I ate too much??? kimchi.

It seems to me some food items are looking to me like separable and tangible items that look to be countable items. Help.

3. When is it more appropriate to put which and when is it more appropriate to put when?

Which/What store/bank do you go to?

Johnney's/ZZZ's

Let's pretend ZZZ is a name of a bank.

4. Please take a look at these sentences and tell me which one is appropriate?

After class, everyone should clean their desk.

After class, everyone should clean his or her desk.

After class, everyone should clean their desks.

5. I saw this sentence and it got me to feel dubious about the underlined part. I think, it is based on the scene where a person with a wife is introducing her to another person.

I would like to introduce to my wife, Patty.

Why not, I would like to introduce to you my wife, Patty.

Again, sorry to ask so many questions in a post.