From the Google AI Blog:
At a very high level, Google Data Search relies on dataset providers, big and small, adding structured metadata on their sites using the open schema.org/Dataset standard. The metadata specifies the salient properties of each dataset: its name and description, spatial and temporal coverage, provenance information, and so on. Dataset Search uses this metadata, links it with other resources that are available at Google (more on this below!), and builds an index of this enriched corpus of metadata. Once we built the index, we can start answering user queries — and figuring out which results best correspond to the query.
When a user issues a query, we search through the corpus of datasets, in a way not unlike Google Search works over Web pages. Just like with any search, we need to determine whether a document is relevant for the query and then rank the relevant documents. Because there are no large-scale studies on how users search for datasets, as a first approximation, we rely on Google Web ranking. However, ranking datasets is different from ranking Web pages, and we add some additional signals that take into account the metadata quality, citations, and so on. As Dataset Search gets used more by our users and we understand better how users search for datasets, we hope that ranking will improve significantly.