as @TheCADWhisperer said: if you can share your design here, it will be easier to help. But, probably, it is best to try to get some basics understood first. For instance, there are several types of geometry that Fusion supports, and each type has its own toolset. You mention Extrude. Extrude only works on Surface or Solid data, and an STL is a mesh. So, that is the first blocker for you. There are ways to convert a mesh to a solid or surface, but they are generally not recommended. A mesh is a collection of thousands of tiny triangles, and if you try to convert to a solid, you end up with an inaccurate solid that is very hard to work with. Depending on the shape of the model, it may be easier to rebuild it as a solid. Or, you may just be better off finding a good mesh modeler to do your work.
Jeff Strater
Engineering Director