The method of migrating smoothy between versions of Sketchup varies for each aspect that you want to update. I can explain some of the ways I do this in a Mac environment. Your results may vary:
Shortcuts: Import automatically, yay!
Extensions: The sketchucation site has been mentioned. There is no batch install from the warehouse. It is tedious but really is best to re download your extensions for the newer version. FYI, almost all current Extensions I use come with a warning about not being compatible with the 2020 version, it’s a stock warning as the 2020 is so new, so far all extensions I have updated seem to be working perfectly. Taking a screen shot of the old extensions is indeed a helpful tip, I set up the screen shot on a separate monitor and just go down the list installing extensions, takes about an hour.
Toolbar setup: We have limited options on the Mac for custom toolbars, and thus far I have not found any reliable way of migrating toolbar setup. I have to recustomize in the new version. I do this by having both versions open so I can see how the old one was set up.
Materials: For materials I do have a way to migrate. I keep all my materials in a separate folder that I link to each new version of Sketchup. When I open 2020 I simply go to Preferences>files>materials and use the icon with the pencil to link that version of sketchup to my materials collection (which is a folder of many other folders which all have materials in them, it must be 3 layers deep like this). Sketchup will load your custom materials on the next startup. If you havn’t already made a custom folder you can navigate to the materials folder from your last version and make a duplicate of that folder, rename it and save it somewhere.
Styles: This works similarly to materials. Make a custom folder somewhere, or just copy over the one from last year and overwrite the 2020 styles folder. One caveat, on Mac at least my custom style folders did not immediately show up even after relaunch. I need to use the option in the styles window to open an existing collection, once for each folder I wanted to import. Once present in the list each collection of styles does persist on relaunch.
Components: Same as styes, relink to a custom folder from within sketchup 2020 or replace the standard components folder for 2020 with your old one. Either way, again, you must use the “open local collection” option from within the component window to open for the first time, after that a collection continues to show up in the component browser menu.