I’m using a Mac as well, so I just tested this out to see if I could figure out the workflow. If you double click the file icon trying to open it in Fusion 360, that doesn’t work. On my Mac, it opens Inventor Fusion which I have an OLD copy of, which will not open the downloaded file, and of course no new version of it exists. I imagine if you don’t have any other viewer on your computer, that explains why it wants to open as text or not at all.
You need to go into Fusion 360 and click the “Upload” button to get that downloaded file into Fusion 360.
I haven’t dug deep into all the features of Fusion360 yet to know if it does any of them exceptionally well, but the idea of having all these functions in one program is something I like. After the past 25 years bouncing between CAD systems, 3D programs like 3DSMAX, Organic/Sculpting tools like ZBrush, and CAD/CAM bridges like MasterCAM, it’s just a workflow hassle moving between programs. Parametric modelers are rubbish for freeform/organic design but once you export a CAD model for sculpting work, you lose any accurate dimensioning you might need to maintain for certain features. The only other way to combine parametric CAD data and freeform/organic is using systems like Geomagic Freeform or CATIA (or other higher end modeling systems) and those are $20k+.