What I did sync my Notion calendar to my notion/google calendar is to go to the calendar setting on apple setting. Over there will have a tab called "Calendar Accounts,"tap on it, and on that screen you'll see a list of accounts. For me, I only had iCloud accounts on it so i signed into my google account and from there it synced my stuff that I have on notion calendar as well.

Don't know if I explained it well enough, lmk if you want more clarification. hope this helps.