I assumed it was China. They both enjoy this activity.

Every single ship in/out of St Petersburg goes via the Gulf of Finland. All those ships will be "Russian" (have stopped in Russia). It doesn't mean they're "Russian". Owner, charterer, flag, crew can all have very different nationalities.

Which part or combination makes them "Russian", in the sense of "the Russian state asked asked the ship to harm Finnish infrastructure, and they actually did it"?

You can lazily speculate about the aggressive, warmaking nation (that illegally annexed Crimea, is currently at war with Ukraine, is regularly sending submarines, ships, drones, jets into the territories of its neighbours) all you like... but if you want to be able to prosecute them, you need to be able to show evidence of the Russian state ordering this action, and that the cable damage was actually caused by that ship. Where is your evidence?