DKIM assigns a validated identifier to a message. It does
DMARC links the DKIM and/or SPF identifier to the domain of the author (rfc2822.From) field AND it can declare the domain owner's preference for what receivers should do with mail that has that domain in the From: field but doesn't get DMARC validation. Receivers are free to conform to that guidance or do whatever else they deem appropriate.
The underlying point here is that these mechanisms work best at identifying valid mail and at letting receivers build up reliable reputations for the domains using these mechanisms.
IMO all of the uses of these mechanisms for identifying bad messages and bad actors represent a near-term, transient artifact, because it is still much too easy for bad actors to "route around" these mechanisms. And the major lesson of the last 20 years of fighting email abuse is that bad actors are very adaptable.
If at first you don't succeed, you are running about average.