Recent Android phones sync a ton of stuff automatically - which I suppose you agree to by signing in with a Google account, but that's also typically required. I know this because on the last two Android phones I purchased, a set of old outdated contacts from my Google account were automatically synced to the phone as soon as I logged in, which I was required to do to begin using the device.

Believe me, I would have opted out of this had I been prompted to do so during setup.

Time to go away from GMail account...

I checked again exactly why this happened: Samsung Calendars app (which is a default calendar app on Samsung phones) has set a default calendar for my new events to my Google Calendar account. And if you just enter the event title and set the time (what one would usually do) - and leave all other settings untouched - then by default it will be added to your Google account which will then be synced to the cloud... You can change these settings (see [1]), but the default is wrong!

[1] https://eu.community.samsung.com/t5/galaxy-s9-series/default...

Be sure that I didn't give any consent...