We call it a market leader.
But we also have pretty strongly enforced law called “kilpailulaki”. Its pretty similiar to antitrust law in the states, meaning it punishes harshly if single company or group if companies use their position in the market unfairly, fixing prices, driving competition away, etc.
Smaller local companies might try to swindle it, but for any big companies breaking it wont be worth the risk.
Monopoly literally means that single entity has exclusive right to the market. Like for example in Finland VR (valtion rautatie, goverments railways) or Alko (only shop that can legally sell spirits for consumers).
There is also term called natural monopoly where getting in to the marked is impossible. Like railways in USA.
Steam is neither.
There is GoG, Epic, itch.io, Microsoft store and like ten smaller stores that are straight competitors for Steam.
Being market leader does not mean they have monopoly and if Steam somehow blunders the goodwill they have with gamers and devs there will be alternatives.
It was a terrorist cell “Maniacs” who blew it up.