I found it to be surprising that this is still not possible after coming to Thunderbird after many years have passed. Currently, there is no option or way to view ALL of ones RSS Feeds in a single Feed. The only choice one has is to individually open each (category) feed and go through it, but its not possible to have all the feeds mixed into a single feed. I may sound redundant but I want to make sure its clear what I am saying.
So far, I feel like the RSS Feed feature in Thunderbird is not useful until seeing feeds All-in-one is possible, and the same goes for making Folders / Categories for feeds. I don't see a point in being able to make different categories or folders, if one does not have the option to see all of its subcategories' feeds in one single feed, or all the things in a folder in one feed.
I believe making categories or folders makes many times more sense if one is able to separate or mix different feeds as one sees fit.
In conclusion and to be more clear, allow mixing feeds via category/folders. For eg. Clicking on a Folder that contains 3 feeds should show a mixed feed of everything in those 3 feeds. Clicking the individual sub-item under a folder should show only the feed for that individual sub-item. The same should be done in general for RSS Feeds.
Thank you for your time reading this I hope this will be considered