Could you give me a source for this?

Preferably a Finnish one. Not by language (although am Finnish so not an issue) but like a reliable source in Finland documenting that’s what they’re for?

I know we do have lots of infra designed “in case of Russian attack”, mainly the direction of roads on the eastern border, but I’d like to read up on this blowing bridges shit.

I mean its also very much usable for other stuff like hanging up temporary cables and hoses and in practice its probably used for things like that more often.

More often? Once you do use them to blow up the bridge then there’s no bridge or hooks.