To play workshop maps in CS2, you first need to find the corresponding section in Steam. (CS2 Workshop on Steam) Steam Community Workshop
There are an incredible number of various works published there, ranging from training maps to single-player games based on CS2. Finding what you need won’t be a problem; however, the most popular maps are listed a bit further down.
Click on one of them and hit the Subscribe button. That’s it! Now this map will be available in your game under the Workshop map section.