The application launches by bootstrapping the root AppModule, which is also referred to as an entry component. Among other things, the bootstrapping process creates the component(s) listed in the bootstrap array and inserts each one into the browser DOM.
Each bootstrapped component is the base of its own tree of components. Inserting a bootstrapped component usually triggers a cascade of component creations that fill out that tree.
While you can put more than one component tree on a host web page, most applications have only one component tree and bootstrap a single root component.