The answer to this can be found in Chapter 4. I cannot provide a page number because I am using my Kindle edition of the book, but Amazon’s searchable copy indicates that somewhere around page 88 in the paperback edition. Look for the paragraph that starts “the first connection is the most direct one…”
Diamond spends more time here talking about domesticated animals than plants. The connection between domestic plants and feeding more people is very clear. He says that selecting only the plants that we can eat results in a situation in which most of the plants on a given area of land are edible. This means we can get far more edible food per unit of land. When we can do this, we can feed more people.
Diamond then goes on to say that domesticated animals feed more people in four ways. First, we can eat them. This provides us with an excellent source of animal protein. Second, they can provide us with milk and other byproducts. These, too, can give us protein. Third, we can fertilize our crops with their manure. This allows us to grow even more food per unit of land. Finally, they can help us cultivate our land. They can pull our plows, allowing us to farm land that we could not previously cultivate.
In these ways, Diamond says, domesticated plants and animals feed more people.