New Android smartphone doesn't synch calendar with old smartphone

Am I the first to post a “Got a new phone for Christmas and something doesn’t work?” thread? Anyway, here it goes: Got a Xiaomi Redmi Note 8 (Android 11) and transferred the data and apps from my old Huawei P8 lite (Android 8) over my Google account. Most things went smoothly (the messages got scrambled and the notes didn’t synch, but I figured it out), but the calendar didn’t transfer to the new phone. I checked the calendar apps on both phones, and they are linked to the same Google account, and I have no clue why they didn’t synch. It’s kinda important to me, because I have all my friends’ birthdays stored. I remember that a few years and three smartphones ago, I transferred the calendar data via a system file, but I don’t know if that’s still possible. Does anybody know what could have gone wrong and how to fix it?

Either your old phone isn’t synching with the cloud, or your new phone isn’t. You write that you checked that they are both connected to the same google account, but did you check that they are both set up to synch your calendar? And are you using Google’s calendar app, or the default one set up by the phone provider?

Assuming you’re using Google’s app:
If you look at your google calendar online, does it show your events? If not, your old phone might have synch turned off for the calendar. If it does, it seems it’s your new phone’s fault.

Yes, I checked if both phones are set up to synch the calendar, and they are. On my new phone, initially only the Google calendar app was installed, but after the synch didn’t work with that, I installed the Xiaomi calendar app, but that didn’t work either. I just logged into my Google account in Chrome on my laptop, and my events also don’t appear in the online Google calendar.