Inventor 2020.3 is an update that lets you work with others who have upgraded to Inventor 2021, using Inventors’ AnyCAD engine. In an assembly, you can reference in 2021 part and Assembly files.
You must have Inventor 2020.3 installed to support opening 2021 part and assembly files. Inventor 2020.3 can be downloaded via the Autodesk desktop app or from your Autodesk account:
For more info on the updates, see: https://help.autodesk.com/view/INVNTOR/2020/ENU/?guid=GUID-E786BB8D-4982-48E2-8219-F0BBF39D7CFC