I'll expand on my comment and turn it into answer.
It is possible that there is a locally running (on your computer) caching DNS server that listens on
127.0.0.1, such as dnsmasq which in turn uses upstream DNS servers to resolve queries originated by your system.
Based on the comment above (by @Gordon Davisson) it is not an out of the box setup on Mac and has to be configured manually.
If your system is able to resolve domain names it is very likely that this is the case and you have locally running caching DNS resolver. Usually they have their own configuration files where upstream DNS servers are specified.
Note: If you use proxy server for Web browsing, your browser will not perform DNS lookups (this will be done by the proxy server), and you will be able to browse even without DNS server being properly configured in your system, as long as the proxy server is configured with an IP-address, not a hostname.