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It’s Match Making isn’t peer to peer, and requires a connection, though. I’d reckon the playerbase to wither quickly without Valve servers and/or Steam friend list, even though technically possible, and a 3rd party could devise a system of finding players.
Valve offers ubiquitous matchmaking services as a part of Steam. They operate as long as Steam does. They’re funded by all the revenue that comes in every time anyone makes a purchase on Steam; these same services apply to third parties who choose to use them, too. Helldivers 2’s servers are run by some function of Arrowhead and Sony, and they shut down when Helldivers 2, in particular, stops making money, rendering the game inoperable.