Based on my limited understanding of DMARC, it serves as the missing link between the publisher of SPF/DKIM records, and the email receiver, by declaring how to use the SPF/DKIM records (subdomains, etc.) and what to do when SPF/DKIM produces a mismatch (quarantine/reject) etc.
The box does this for you. If you’re using your own DNS provider and not your box (i.e. external DNS), it tells you what records you need to set in the listing in the External DNS tab of the control panel — there’s nothing else you need to do.
@ionos’s explanation of what DMARC is for is spot on.