This looks perfectly normal. In Catalina and Big Sur, there are two separate volumes, called (by default) "Macintosh HD" and "Macintosh HD - Data". The first contains the OS and the second all the applications and user files.
The Finder, however, hides these details from view, and presents the user with what appears to be only one volume, as was traditionally the case.
Nothing wrong there. Now, your drive is very small, and it may really be full. It could also be the case that there are a lot of purgeable space.
In Finder, select your drive and press Command-I to open the Info window. What does it say, regarding free and purgeable space?
It will look similar to the image below. It remains to be seen if the available space includes a lot or "purgeable". If that is the case we can try to reclaim some of it.
