That’s the thing, there is no difference between categories and forums in Discourse. Everything is a category. In fact if the setting wasn’t already turned off that allows it then it’s possible to have threads that don’t have a category at all. Discourse uses categories the way blogging software does rather than the way more traditional forum software does.
sub-sub-categories are possible, but it requires a plugin to do it. I use custom built Docker images for the Discourse instances I admin that include a plugin for it and enable it by default. I’m so used to dealing with those that I forgot it wasn’t part of the core
Discourse has a bit of learning curve initially, but once you get used to how the parts work it actually makes sense. I think my favorite way of explaining it was what a friend told his site’s members “Think of it as a Wordpress blog with a really bizarre template that everyone is allowed to create posts on.”