What is / Full version
DKIM
The public key lives under _domainkey. s= is the selector and d= is the signing domain. A valid signature is not, by itself, DMARC alignment.
Why a selector exists
The key namespace is subdivided with selectors so a signing domain can have more than one public key at the same time. A selector might name an office, a month, or a partner that is allowed to sign for a while.
Periods in a selector are DNS label boundaries. The selector march2005.reykjavik is more than one label. That is how a domain can delegate part of the selector namespace.
The query
All DKIM keys are stored in a subdomain named _domainkey. Given d=example.com and s=foo.bar, the query is foo.bar._domainkey.example.com. The base specification stores the key in a TXT record.
TXT strings are concatenated with no extra whitespace. More than one record in the set for a selector makes the result undefined.
A signature is not alignment
Any domain can put a valid signature on a message, including a mailing list or anyone else. RFC 7489 says a valid signature is not enough to infer that the From domain is authentic.
DMARC uses the d= domain, and only an aligned one counts. The short DMARC page is the policy. This page is only the key.