In Moqui XML Screens all XML elements are implemented using FTL macros. For HTML output you can see the default (OOTB) macros in the runtime/template/screen-macro/DefaultScreenMacros.html.ftl file. The XML element used to include sections of other screens is section-include. Because there are no Java/Groovy classes that handle XML screen elements you can see everything in the macros.
In this case the Moqui API call in question is on the ScreenRenderImpl class which is available in screens as they render in the 'sri' variable. Here is the relevant line from the screen-include FTL macro:
${sri.renderSection(.node["@name"])}
The renderSection() method takes a String argument with the name of the section in the current screen or the fully qualified screen location plus a hash and the section name as in your example.