Ever since macOS Catalina (10.15), Apple implemented a separate appstored (daemon) and appstoreagent processes, which are I believe responsible for handling files and downloading in the background.
In contrast to Mojave and earlier, you will now see an additional folder while downloading files from the App Store. Once you click GET or UPDATE and run the following command, you can confirm having two instead of one App Store related folder:
ls -d $TMPDIR../C/com.apple.appstore* /var/folders/x3/h4gbhgx52mx_wv30p8m0z9vh0000gn/T/../C/com.apple.appstore /var/folders/x3/h4gbhgx52mx_wv30p8m0z9vh0000gn/T/../C/com.apple.appstoreagent
This agent folder is the newer location where the temporary subfolder resides for preflight and downloading the actual package ; until installed.