Diamond lays out, in Chapter 1, the orthodox view of the peopling of the Americas.
He argues that the first Native Americans came over from Siberia to Alaska. They came at a time when sea levels were much lower than they are now. They might have come on land when there was a land bridge. Otherwise, they might have come by boat when there was less of a gap between Siberia and Alaska.
From there, the Native Americans came down from Alaska into the rest of the Americas. They did so by land when there was a ice-free corridor through the ice sheets that covered Canada.
It should be noted that this view is not necessarily shared by archaeologists today.