I recommend avoiding online CAD platforms like Onshape and Tinkercad for any serious work, as they don't allow you to export the design (extrusions, dimensions, sketches, etc.), so it cannot be archived or edited elsewhere. You can only download the final computed mesh rounded to some fixed precision, which is suitable only for printing. When the website inevitably shuts down, the design will be lost forever.
With standalone CAD packages, you'll be able to convert your design to other formats or use the original software in a virtual machine, even 10, 20, or 50 years from now.
