I added the potential topic of this post to the title, a title like "hello", "help", or "a question" doesn't help others understand what you are talking about, and asking a question or introducing a topic at a first impression makes it that little bit easier for people trying to help. This (and other recommendations and advice) is also stated in our discussions guidelines.
If it is about categories specifically in the discussions/feeds page, it's the link that Andrew provided.
I don't know what it looks like on mobile, but on desktop, when you first open Discussions, it should show you a list of threads. Just above the left side of the list, there should be a dropdown list that let's you pick which category to see. That is the filter. Admins should see an "edit categories" option at the bottom of that list.