> Which is only the core problem because dmarc fixed the other core problem of figuring out who the given sender is.

Does it verify the sender or the domain/service which the sender is using?

Yes. If an email comes from alice@gmail and validates to gmail, alice sent it.

It's possible that gmail screwed up and gave Bob access to Alice's account. In this situation, though, Alice still sent it.

Google is not the only email provider in the world, though. Not all email services are global corporations, and we must be careful never to interrupt the services of independent email providers.

If it comes from alice@foobar and validates to foobar, Alice sent it