Each version of SketchUp installs as a totally separate application so each has its own Scrapbook folder. Your custom scrapbooks for LO2020 should be under ~/Library/Application Support/SketchUp/SketchUp 2020/LayOut/Scrapbooks. You can copy those and paste them into the equivalent folder in 2021. You can see that path in LayOut’s Preferences>Files panel.
Alternatively you could make LO 2021 look at the LO 2020 Scrapbook folder by adding the path in Preferences>Files. I prefer to copy the Scrapbooks, though.
I expect the default location (as above) would be considered the preferred location. The location is normally not something you need to access on a regular basis so the files are safe from inadvertent overly zealous cleaning. Since it’s the default location, there’s no worries about losing the path if you have some reason to reinstall SU/LO. Still, you can choose a different location if you wish. Maybe put it in Documents.
As for the next version finding the location automatically, no, at least not currently. You would need to go into Preferences>Files and choose your non-default path.
Which scrapbooks are you opening? The ones supplied with LO or your custom ones? Is this the message you are seeing?
That just means you are opening a file that was created in an older version of LayOut. Like the LO templates, scrapbooks are just .layout files like your project files.
When you create your own custom scrapbooks, distribute the content on the page but keep it relatively condensed. Make the page size only large enough to fit the stuff. That will make the items show larger in the Scrapbook panel. Here are a couple of examples. You can see how the page is sized to fit fairly closely to the content.
This one shows the items quite small in the Scrapbook panel. I knew that would happen but the Large Toolset tool bar on the left dictated the layout. And I can make the Scrapbook window taller to make the individual tools easier to pick out.
These break lines are also on a small page and with the arrangement they all show quite large in the Scrapbook window.
You can use Scaled Drawings for items in your scrapbooks so for things like scale bars you set the scale to match the viewport.
The reason I prefer to use the default location for scrapbook is because I also use the default location for my templates in both LO and SU as well as styles, materials, and my local collections of components. They are all basically in the same place so after I install a new version, I just copy those folders from the previous version and paste them into the appropriate locations in the new version and I’m finished. Probably less than a minute to make all that stuff available in the new version and since I’m copying the folders, I still have the original folders as backups just in case.