How to add sub-forum drop down menu to categories

[PracticalBot]

I've seen an XF forum with a sub-forum drop down menu within each category listed on the main page as the red arrows and box show in this image:

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I redacted the names etc since it's not my forum.

I have XF 2.1 and I want to have these sub-forum drop down menus in my categories on the main page. Is there a way to do that? I was told it is native to XF but I cannot find it anywhere.

Thanks in advance for any help.

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