(Just to add to what bean was saying)
If you are importing a STL in Fusion, normally the flow is:
1- Import the STL as a mesh
2- Edit the Mesh as needed (you can cut, fill, do bunch of modifications)
3- You can export as a stl if you have nothing else to do.
4- For more advance modifications, you convert the mesh to a body (Mesh to BRep)
5- Merge bunch of triangles to get uniform surfaces.
6- Modify the body with more advanced tools to get what you are looking for.
Example:
I got the stl for a probe holder
Now there is something written on the handle and I want to remove it.
I simply select the text by painting over it.
Use the remove/fill tool and completely delete the text.
Now I convert the mesh to a body (Mesh to BRep).
I use merge to get rid of some triangles.
Now it's trivial to split the body in multiple one.