It used to be that Most Web Applications on the web were MVC Apps. But in the Past few years there are many apps that do not have SEO concerns and opt out to be designed with SPA frameworks like AngualrJs, React in my mind. Many of these applications happen to be line of business Applications that are gated and are not publicly available. The Irony is these Applications are being designed with almost identical Network Architecture as their previous MVC counterparts.
Let’s review how most enterprise level MVC apps worked. There would normally be an Application Website and a separate WebApp for handling things such as authentication. So for instance you might try to use an expense management software that is hosted at https://www.awesometimemanagment.com/ however, the first time you login, you would be redirected to https://login.awesometimemanagment.com/ because you lack the proper session cookies that the original WebApp could identify you with.
Figure 1: Typical Data Flow for an MVC App
Figure 2: Typical Data Flow for an SPA App