http://o54hon2e2vj6c7m3aqqu6uyece65by3vgoxxhlqlsvkmacw6a7m7kiad.onion/en/blog/2021/3/3/mullvad-doh-and-dot-beta-release
This is the case for every URL you visit, every file you download, and every image that loads on a website, including ads. Since your browser contacts the DNS server for each and every new domain name request, the DNS server will know which websites you are looking at and links from that pages to other resources (like ads).