DMARC sits on top of SPF and DKIM. Those two can pass on a domain that is not the From address. DMARC asks whether the authenticated domain aligns with what the recipient sees, then applies p=. Without DMARC, SPF and DKIM are an honour system.
The record lives at _dmarc.yourdomain.com. One TXT. This checker follows a CNAME if you pointed _dmarc somewhere else, then reads p, sp, pct, rua, ruf, and alignment.
none is not protection
A valid p=none record is still a monitoring feed. Spoofed mail gets delivered. That is the right first week, and the wrong third year. Use rua reports to find every sender, fix SPF/DKIM for the legitimate ones, then quarantine, then reject.
Check SPF and DKIM on the same domain with the SPF checker and DKIM checker. The longer walkthrough is the SPF, DKIM, and DMARC guide.