Difference between full stop and comma usage when writing dialogue?

I'm writing me second novel. I know, I know. The first one is full of mistakes. But I'd like to do better this time. Can't get my head around the use of commas and periods in dialogue. When I look at Kindle samples for popular novels the rules seem confusing.

For example please explain or correct this…

James looked up at Alfred. "Time?"
"Haven't seen it yet." The Instructor tapped his foot on a red line.
"Well boo hoo," Claire said, "I can't even see the sky."

The dialogue might not make sense. I've essentially copied the punctuation from a Kindle sample that's rated pretty highly. I didn't want to risk offending the author. I would have thought that it should have been written like this:

James looked up at Alfred. "Time?"
"Haven't seen it yet," the Instructor tapped his foot on a red line.
"Well boo hoo," Claire said, "I can't even see the sky."

Note the comma after yet and the non capitalization of the T in the.

Which is correct, or how should it be written?
Added (1). As per the advice given here, I'm looking at some of the published Amazon novels now that I've bought. In a very very popular novel I'm quoting thus:

"She'll be okay, she's a strong one," Calvin says. He holds Lilly's gaze, "How about you?"
then
"I hear ya." He cocks his head a bit, "You're pretty handy with that firearm."

Why is the "I hear ya." finished with a full stop and not a comma? If the comma is correct after strong one, then why does How about you begin with a capital?

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