In all Hadoop files, the * -default.xml files mainly serve as documentation for the default values. The default values are all hardcoded in the source. The * -site.xml files are any site-specific changes and override the default values specified in the * -default.xml files.
To a specific point, yarn-default.xml and yarn-site.xml are both read by the Hadoop YARN daemons, with yarn-default.xml representing default values and yarn-site.xml representing user configuration values that override values in yarn-default .xml.