Here is the link to the office. documentation - https://support.google.com/calendar/answer/37111?hl=ru
To export - Click on the cog top right in the opened window choose "Import and export on the left". Get the file ics files.
I understand that they no longer support xml.
Here, Converter to CSV format
Here, a universal Converter
As for plugins, it is necessary of course to check. Here, the list is