Thanks for the kind words, Ravi. :-)
There are a couple of options that spring to mind. The first is to draw the container diagram and simply use colour coding or notation to signify the containers that make up a sub-system. For example, "all red containers are part of the recommendation sub-system". Another option is to create a separate C4 model for each sub-system so that you only focus on one sub-system at a time. In other words, you would draw context, container and component diagrams for each sub-system. For example, the container diagram for the recommendation engine would show the containers for the recommendations engine, and all other sub-system interactions as black-box interactions. Hope that helps!
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