Figure 2.1 depicts Model I. Here, we imagine an economy that produces only consumption goods. To keep Model I as simple as possible we further suppose that the only consumption good is cars. These cars are produced by firms which are staffed by the households and owned by the households.
To put it in the language of economics, the two factors of production are labor and capital, and the households own both of them. There is no role for government or for the rest-of-the-world in Model I, so these sectors are omitted.