I have crossed numerous borders in the past few months and wherever proof of vaccine was needed, it was done prior to travel.
Finland is different. The airline requested nothing regarding proof of vaccine. However, after landing in Helsinki, and walking towards the exit which is past baggage claim, you are required to show your proof of vaccine before you can leave.
I showed my CDC card and was waved through. She didn't even turn it over to look at the booster.
Once in Finland, all Covid rules are gone.