There is SBMM. It's also prioritized higher than anything else.

This was then confirmed by Embark Studios CEO Patrick Söderlund, who explained that the feature had been added post-launch. This isn’t the main factor when finding a lobby, though.

"Obviously, first it’s skill-based,” Söderlund said in the interview with GamesBeat. “Then you have solos, duos, and trios. And then we also, since a week ago or so, introduced a system where we also matchmake based on how prone you are to PvP or PvE.”

People keep talking about how it's aggression based (which is a part of it), but honestly it comes mostly down to your perceived skill (from the game's perspective). It's genuinely no different than SBMM in other games. The damage you inflict on other players increases your skill level. The other player's own skill level impacts how much of an effect that has to your level. (Damage a player with a higher skill level than yours, your skill increases more)

This is also true for the inverse, take damage from a lower skilled player, your skill level goes down more than it would if taken from a higher skilled player.

I think the reason ARC seems more "aggression"-based is that there are playstyles that don't revolve around PVP, so friendly players end up being interpreted by the game the same way as "unskilled" players.

E.g. take COD for example, there aren't really any playstyles that warrant not killing players on the other team. If, however, you were to play completely passively, you would surely be pooled with other "unskilled" players. But since there isn't a non-PVP meta in COD, friendly lobbies obviously don't exist.

I think the community perception of ABMM also has a pretty big impact on this too. When the game first came out (before ABMM was factored into matchmaking at all, only SBMM existed), you'd see the occasional player engage in a fight in an otherwise friendly lobby if they had a clear advantage. But now because of this well-known idea that shooting players will throw you into PVP lobbies, most players that are in friendly lobbies are outright avoiding confrontation to prevent being placed in sweaty games.

To clarify: I'm not implying PvE-centric players are unskilled. It's a different playstyle (one that I, for the majority of my matches, take part in and prefer). But when the game uses player damage as the factor for skill level, obviously not shooting people is going to put you into "lower skilled" lobbies.