Quick Match can be considered "casual MM" but it does use an internal MMR to put you up against "similarly skilled" players.
Quick Match:
No drafting phase (picking heroes), the engine puts together teams, usually worse than the kinds of teams players make but they try to pit two similarly capable teams against each other
Atmosphere is more relaxed
No visible rating or whatever so the pressure is lower
Duplicate heroes on both sides
Free rotation heroes available every week
Hero League:
Drafting phase (teams take turn choosing heroes, no duplicates)
Atmosphere is more tense
People gravitate toward what works well in any given meta so compositions are kind of stale
Visible rating so you'll feel a lot more pressure to perform
No free rotation available, you need to have heroes unlocked via gold or cash to play (at least 10), and you'll want to have a flexible stable so your team isn't stuck with no healers or no tanks
That said, it doesn't mean the players you see in QM will all be worse than the ones you see in HL. People who take competitive very seriously will gravitate toward HL but the upper echelons of QM will have players as skilled as the ones in the upper echelons of HL, only, you don't exactly know when you're there because the MMR is hidden. Sites like
https://www.hotslogs.com can try to model your hidden QM MMR using their own algorithm and it affords a reasonable approximation of what bracket you're being matched up against.