Every teacher individually?
AFAIU, yes. (That being said, while I have teached at the university level in Finland, I have no experience of the Finnish primary and high school system from the faculty viewpoint, so you might want to double-check with someone else). Also, consider that there are something like 5 million Finnish speakers, so it's not a particularly large market, so teachers are not exactly going to be overwhelmed by the number of available textbooks. E.g. in physics I think there are about 3-4 book series covering the high school curriculum. I suppose it's a bit different in the US, where one presumably cannot assume a teacher has time to evaluate all the available textbooks. Then again, at least from over here it seems that textbook selection in the US is extremely politicized (can a biology textbook cover evolution? WTF!?) which probably isn't conductive to a good outcome either.
Textbooks must teach to the content of the abitur and the standards being established by the Bologna Process. So, I guess the curricula are well defined. But I'm still surprised that this decision would be left to every teacher individually.
Yes, the Ministry of Education defines (broadly) the curriculum, so it's not like teachers are allowed to teach whatever they fancy. But generally, the large degree of autonomy given to teachers is often seen as one of the reasons why Finland does so well in these PISA tests. Teachers over here are pretty well educated, and it's a well regarded profession. Of course, there are other reasons as well, e.g. Finland is culturally pretty homogeneous and there are quite small socioeconomic differences compared to many other countries. Anyway, it's not like teachers are alone in choosing textbooks, of course they talk with colleagues etc., and professional societies do from time to time publish reviews of the available textbooks, which I assume teachers read carefully.
As an aside, the Bologna process AFAIK covers only higher education (at the polytechnic/university level, bachelor/master/Phd), not high school. Of course, it indirectly covers lower education as well in the sense that it effectively requires that students entering higher education have certain skills.