I think they have to be doing some player-skill-based matchmaking in those queues. I think it would just be really un-fun if they didn't, and I'd bet Arena is optimizing for player hours over anything else. Nothing ends an Arena session like getting dunked on multiple games in a row in a ranked queue lol.

I’d bet that each format – Standard, Explorer, Alchemy, etc. – has its own set of weights.

I never thought about that but I'd bet you're right and I'd hope they're getting updated a lot more regularly than the Brawl ones.