In Helsinki, as at all Schengen airports, the following, somewhat confusing, pattern applies:

Arrivals from a Schengen country: you will deboard directly to the Schengen gate area. No security control needed. Immigration only if connecting flight is to a Non-Schengen destination. Comparable to US-domestic connecting procedures.

Arrivals from a Non-Schengen country:

a) Arrivals from the UK and Ireland: you will deboard directly to the Non-Schengen gate area. No security control needed. No immigration needed if connecting flight is to a Non-Schengen destination. If connecting flight is to a Schengen-destination, you will go through immigration to access the Schengen area gates.

b) Arrivals from the rest of the world: you will deboard to an isolated area, where you will proceed either to immigration -> baggage claim -> customs -> landside or, if connecting, you will proceed to security control. After security control, you reach the Non-Schengen area w/o immigration, but to reach the Schengen area gates, you will go through immigration.