As CLIs grow it can be useful to nest commands within topics. This is supported simply by placing command files in subdirectories. For example, with the Salesforce CLI we have a topic sf config with commands like sf config set and sf config get. The directory structure looks like this:

package.json
src/
└── commands/
    └── config/
        ├── index.ts
        ├── set.ts
        └── get.ts

The help descriptions will be the description of the first command within a directory. If you'd like to customize the help description, add it to the package.json like so:

{
  "oclif": {
    "topics": {
      "apps:favorites": { "description": "manage favorite apps" },
      "config": { "description": "manage heroku config variables" },
    }
  }
}

Subtopics can be created by making subdirectories within topic directories, but for UX reasons we generally discourage going more than 1 or 2 levels deep even for the largest CLIs.