Just to add to that answer, I’d the biggest difference is the grim powerlessness. The players can barely delay the inevitable and most will probably die in the process.
There are way more skills aimed at investigations, and the goal are often more aimed towards a McGuffin that solves the issues, or sometimes the goal is simply to just be able to leave in one piece and hope someone else cleans the mess 😅. (There’s actually a fan story adapted into a RPG called « Delta Green » where you play a secret military tasked with dealing with mythos threats)
As D&D is the epic hero’s journey where Gandalf saves everyone at the last minute, CoC is the horror thriller where the protagonist dies, only a rando side character survives and walks off screen as the dead monster opens its eyes again just before cutting to black.
I mean… there can’t be an ongoing genocide if they are all dead taps temple
(God I hate this planet now…)