Using Classic Outlook on Windows for personal email, calendar, and contact management
This link solved my problem, which I thought to be the same as in this threat (apologies if not) : my Outlook calendar account didn't synchronise with my Google calendar account. Both on phone (Apple) and in browser.
Via this link, I was able to select the outlook account (which was not selected at first), and it started to sync. According to my findings, it syncs every 3h (so not instantly).
Hope this helps! If not, sorry but it worked for me :)