Using Classic Outlook on Windows for personal email, calendar, and contact management
I've found this solution https://support.google.com/calendar/thread/205900609?hl=en&msgid=210862726
Which was also posted here, earlier by Hip Pickles7 (I can't find how to get a link to that comment..)
If you have the outlook app installed on Android, open the app, open settings, click your outlook account, and ensure 'Sync calendar' is turned on.
This synchronizes my Microsoft calendar with my Google calendar. I need to share my Microsoft calendar with another person who uses a Google calendar.