I'm a blender beginner. I have an .STL of an anatomical figure and I need to crop a small chunk of it and export it in a separat .STL file, but so far I haven't found anything quite relevant on the web. Since I'm a blender-nobb, I figured out I don't even know what to search for exactly.
Have you tried looking into the Boolean modifier?
If you object is a closed, watertight, manifold mesh you could use Boolean operations to break it into chunks of the desirable parts.
Just create a new closed watertight mesh object that as a shape or volume that engulfs or includes the part of the STL you want to keep and use it in a Boolean operation to intersect with the existing mesh and extract the part you want.
You can also try the Knife Project operator if a simpler cutting or slicing of the original geometry in one direction will suffice to obtain what you want. You can use something as simple as a plane or a bezier curve to cut your geometry in the desired way.
Edit If your object is already composed of separate parts but all joined in a single object you can simply enter edit mode, select them all press P to separate them and select the option All loose parts to divide them into different objects.