There is one pinned to each category. You will see that one topic at the top of the category (until you unpin it for your account). You will also see the newly created "How to get the best out of this forum" topics in the parent category listings and the aggregate topic listings (e.g., "Latest"), but they are only normal topics there, not pinned, and so they will be replaced in those listings by more active topics as usual.
You will recall that I proposed a major reduction in the number of categories. However, there was not a sufficient level of support from the forum community to justify making such a significant change. So I instead went for a less dramatic change (as proposed by @sterretje, thanks!) that focuses more on rearranging the existing child categories under a logical set of top level categories. This work will result in the removal of multiple superfluous or redundant categories. I have already removed six categories (done in a manner that does not result in the loss of any topics or breakage of any links). A few more categories will eventually be removed (per @sterretje's proposal). The removal of a category requires the manual recategorization of all topics in the category, which is a lot of work, so this will take some time.
I did create the following new official product categories (per @sterretje's proposal):
The Arduino company has specified that there should be a dedicated category for each of the active official products. These categories were created sporadically and so we ended up with a situation where we had categories for only a somewhat random list of the official products, missing some of the most significant boards. My opinion is that, if we are going have these product categories, they should be reasonably comprehensive (I would refrain from creating categories for products that aren't much discussed here unless explicitly directed to do so by the Arduino company management).
While the restructuring of the product categories was in process, I actually received feedback from multiple people who noticed the absence of categories for products like the UNO (example), even though we never had such categories before now.