The best way to learn the structure of Metasploit is to browse through its directory. When using a Kali Linux, the Metasploit Framework is usually located at path /usr/share/metasploit-framework, as shown in the following screenshot:

At a broad level, the Metasploit Framework structure is as shown in the following screenshot:

The Metasploit Framework has a very clear and well-defined structure, and the tools/utilities within the framework are organized based on their relevance in various phases of the penetration testing life cycle. We'll be using tools/utilities from each of these categories as we progress through the book.
In the next section, we'll have a brief overview of all the Metasploit components.