RFC 2818 describes two methods to match a domain name against a certificate - using the available names within the subjectAlternativeName extension, or, in the absence of a SAN extension, falling back to the commonName. The fallback to the commonName was deprecated in RFC 2818 (published in 2000), but support still remains in a number of TLS clients, often incorrectly.
Docs: https://tools.ietf.org/html/rfc2818 https://cabforum.org/baseline-requirements-documents/