Each major version of SketchUp installs as a separate program. Plugins and extensions won’t be transferred over. The best practice is to install the extensions fresh from their sources to make sure you have the most recent and up to date versions of them.
Use the Extension Warehouse panel in the Window menu to install extensions you got from the EW before. If you have extensions from Sketchucation, download and install the Sketchucation Plugin Store tool and use it to install extensions from there. If you used that tool with SU2018, you can use the tool to install your bundle or extensions. This only works for extensions from Sketchucation and only if you used the tool to install them before.
When you install SU2020, make sure you right click on the downloaded installer and choose Run as administrator from the context menu.