How does Sam Harris take into account people being born or dying in the moral landscape?

Sorry if this seems dumb or dull.

From what I seen Sam Harris’ view seems to be a slightly utilitarian, the right choice is the choice that increases happiness in the long term. So you could cut open a boys stomach, if in the long term that would increase happiness, for example an appendectomy.

But how does he measure happiness? If it’s average happiness, then it would be ok to kill unhappy people. If it’s total happiness then it’s ok if everyone is unhappier as long as there more people.

If death seen as the ultimate unhappiness, so euthanasia is a bad moral choice?

And on a neuroscientist level what is happiness?