You write a catalog, an XML document, that maps from the identifiers that you use to the actual resources you want to use:

<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
  <system systemId="https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
          uri="/share/dtds/xhtml1-strict.dtd"/>
</catalog>

The “system” element maps system identifiers. In this case the XHTML1 “strict” DTD is mapped to a local file on my laptop, /share/dtds/xhtml1-strict.dtd. (There are other elements for public identifiers, URIs, delegating requests to different catalogs, doing simple string substutions, etc.)

The resolver accepts a list of catalog files, so you can have different catalogs for different projects and mix-and-match them. You don’t have to write one monolithic catalog!