Layers and components should be distributed across separate physical tiers, but only where it is necessary to do so. There are several reasons for implementing distributed deployment and that includes security policies, physical constraints, shared business logic, and scalability.
For the sake of simplifying this book's content, and for the needs of the TicTacToe application, we are only deploying the application as a single instance, not distributed (we will cover this in Chapter 12, Hosting ASP.NET Core 3 Applications). Therefore, suffice to say that there are other ways and means of hosting an application with multiple instances as a distributed application.
In web applications, you should deploy the business layer and presentation layer components on the same physical tier to maximize performance and ease operational management, unless security restrictions need a trust boundary between them.