The US Stock Market in 2025 and its relation to Japan Time

What is the US Stock Market like? I guess it's something like this:

Trading Hours:
Normal hours from 9:30 to 16:00
From 4:00 AM to 9:30 AM
After 16:00 until 20:00

This is all in Eastern Time (US).

Main exchanges? New York, NASDAQ, American. But there may be others.

Rules and such:
The currency is dollars. You can buy from 1 share.
The settlement is in 3 days. The fees vary.

Time difference with Japan:
In summer, 13 hours; in winter, 14 hours. It's complicated.

Day off:
New Year's Day, Martin Luther King Jr. Day, President's Day...
There are a few others, but I can't remember them all.

What's interesting is that sometimes things end early before holidays. I wonder why?

Exchange Features:
It seems that New York is the largest.
The Nasdaq is a pioneer in electronic trading. Are there many tech companies?
American companies tend to be smaller.

It's quite complicated, but I feel like I somewhat understood it. However, I'm still a bit unclear about the details.