I noticed that there are microfilms related to a certain area that are only viewable and reachable through the URL.
They do not appear among the associated microfilms if you search for the location in question, nor if you search directly for the microfilm number.
Is there an index with all the microfilms present on FamilySearch with relative description?
All microfilm has now been digitized. If the images have been added to the site in about the last two years, the catalog has been locked for a revamp, and the DGS may not appear in the catalog.
If the images are not restricted from viewing, you may find them in the Images Search section, under the DGS number or by location.0
There have always been films (or image groups, nowadays) that haven't (yet) made it into the catalog; as Áine mentioned, anything newer than mid-2021 definitely falls into this category. My past experience is that eventually, they show up, but it takes years.
(The situation with Hungarian civil registrations used to be bad enough that I have a copy of a large spreadsheet that the genealogy community compiled, with film numbers and their contents; people found them by editing URLs and seeing what they got. As far as I can tell, all of those films are now correctly listed in the catalog.)1