Autodesk is only able to get away with what it does because regulators allowed it to buy out its competitors. Further, the extreme technical difficulty of reverse-engineering complex file formats--including legal obstacles such as enforceable shrink-wrap contracts against reverse engineering, the DMCA ban on reverse engineering, and patent issues--means that it's virtually impossible for competing, interoperable, products to emerge in Autodesk's markets.
The main competition for Autodesk's Fusion 360 is SolidWorks. It's possible, although not that easy, to get models back and forth.[1]
There's FreeCAD. There are many low end 2D CAD programs. If you're building something bigger than a house or more complicated than a keyboard, though, you probably want a pro product.
[1] https://www.buildercentral.com/solidworks-vs-fusion-360/