in my experience it's just a headless browser with a bypass-paywalls extension
There have been periods of weeks/months when they don't have paid access to those Finnish sites. Tried it just now on a hs.fi paid article from today and it didn't work, but for example paid articles from just a week ago seem to have been captured as a premium user.
It is curious how they have time to do it and I wonder if news sites of other smaller languages get similar treatment.