Hi Aroldo,
In DSpace, the "most relevant" ordering always corresponds to the relevance "score" provided by Apache Solr. Apache Solr will attempt to rank or score results based on your search query, and will sort them so that the most relevant results are more likely to be at the top.
More information on Solr relevance can be found at
https://solr.apache.org/guide/8_0/relevance.html
Basically, DSpace's search engine is entirely driven by Solr. DSpace does not have its own relevancy score/rank. DSpace will just return the results based on how your Solr is configured.
Tim