I just loaded Sketchup 2020 and when I look into the extension manager I cannot call up any of my previously acquired extensions. With previous upgrades my existing extensions have been available in the Manager.
Updates within the same major version would keep plugins. So, going from 19.0 to 19.1, 19.2, 19.3, would keep them around.
Going from 18 to 19, or 19 to 20, does not copy the plugins across. You can do that manually, but there may be some extensions that have been updated to be compatible with the new version. The safer way to do it is to open Extension Warehouse inside SketchUp, and choose My Downloads, click on the title of an extension you want to install, then on the Install button.
When the Extension Warehouse was changed recently, some extensions may not have been carried over from the old My Extensions list to the new My Downloads list. For any that are missing from the list you would need to use the Find option to get to the extension, then install it.
If you decide to copy plugins manually, the 2019 ones are in this folder:
%AppData%\SketchUp\SketchUp 2019\SketchUp\Plugins
and you would copy them to here:
%AppData%\SketchUp\SketchUp 2020\SketchUp\Plugins
If you see error messages when you open SketchUp you would need to remove the extension that is showing an error.