Following on from a Container Diagram, next you can zoom in and decompose each container further to identify the major structural building blocks and their interactions.
The Component diagram shows how a container is made up of a number of "components", what each of those components are, their responsibilities and the technology/implementation details.
This is an example Component diagram for a fictional Internet Banking System, showing some (rather than all) of the components within the API Application. Here, there are two Spring MVC Rest Controllers providing access points for the JSON/HTTPS API, with each controller subsequently using other components to access data from the Database and Mainframe Banking System.
See BigBankPlc.java for the code, and https://structurizr.com/share/36141#Components for the diagram.
Please note that, in a real-world scenario, you would probably want to extract components automatically from a codebase with the component finder, using static analysis and reflection techniques.