In SU2020 Pro (Mac) I clicked on the update menu item. Downloaded and installed the 2021 Mac version. Now I have both 2020 and 2021 applications. Is there a way to transfer my 2020 configuration and extensions to the 2021 version? Or did I do the update incorrectly? The 2021 version does not include any of my extensions or configurations.
Thanks for your help …
They are separate installations. Shortcuts now copy over automatically, but you should re-download the latest versions of your extensions and install them fresh.
Don’t bother with ones you rarely or never use, until you next need them.
Configuration details like tool palette and custom templates don’t carry over either, IIRC.
But you can ‘point’ your new installation to saved files, materials etc. However, the Templates folder location is not editable.
In addition to what John said, in Finder you can use the Go menu, Go to Folder…, and paste in this path:
~/Library/Application Support/SketchUp 2020/SketchUp/
In that folder you will see Templates, Components, and Materials. As those are usually empty with a new installed version, you could select and copy all three folders.
Then go to this folder:
~/Library/Application Support/SketchUp 2021/SketchUp/
and paste the three folders, replacing the ones that are there. That will then make your 2020 custom collections show up in SketchUp 2021.
Thanks John and Colin (especially the folder links !) 
Advice duly noted.
Is the upgrade procedure for Windows the same? As in, customization is not automatically transferred?
And my bad, I should have researched the transition more fully. I had spent a lot of time and effort to get 2020 configured to meet my requirements. Even more time was spent in installing, maintaining and customizing an extensive list of extensions. Now I am faced with re-installing and re-configuring the extensions. Some of the nuance changes have been lost to time and I will have to stumble across these as they come up as problems, again. The minor differences in 2021 were not compelling for my requirements and in hindsight I should have bypassed the upgrade. Maybe a tad sensitive to the upheaval in my SU work routine, but dislike spending time solving problems, twice. 
On Windows you can use File Explorer in a similar way to Go to Folder on Mac. Copy and paste this path into the path field of File Explorer, to get to the 2020 folders:
%AppData%\SketchUp\SketchUp 2020\SketchUp\
and this to get to the 2021 folder:
%AppData%\SketchUp\SketchUp 2021\SketchUp\
Hopefully there will be enough improvements in the 2021 version to save you some time over the next year. At least as much time as you spent setting up 2021.