This video explains how prepare .stl and other file formats containing triangulated, high polygon count meshes of irregular or organic shapes for use in Fusion 360.
The current tools in Fusion 360 require a user to lower the resolution to under 20k faces and then convert the mesh directly into a solid body ( BRep), which often results in inferior quality.
However, there are a number of other tools available that when used in conjunction with Fusion 360- can be used to great effect in creating hight quality surfaces and solid bodies that can be edited with most of the modeling tools available in Fusion 360.

Autodesk Meshmixer:
http://www.meshmixer.com/
InstantMeshes:
https://github.com/wjakob/instant-meshes
Blender:
https://www.blender.org/

The online sources for the two .stl meshes used in the video are:
Terrain mesh :
http://touchterrain.geol.iastate.edu/
Head sculpture:
https://www.artec3d.com/3d-models/hea...