How does Maven resolve plugin versions?

Every version of Maven binaries has certain versions of plugin versions hardcoded. That's to make a somewhat reproducible build in the cases when user doesn't provide his own version information. Which you are encouraged to do and once you populate <pluginManagement> section with the plugin versions of your choice, the build will start using it.

Maven defines 3 lifecycles in META-INF/plexus/components.xml:

1. Default Lifecycle

Default lifecycle are defined without any associated plugin. Plugin bindings for these lifecycles are defined separately for every packaging in META-INF/plexus/default-bindings.xml

e.g. Plugin bindings for jar packaging

<phases>
  <process-resources>
    org.apache.maven.plugins:maven-resources-plugin:2.6:resources
  </process-resources>
  <compile>
    org.apache.maven.plugins:maven-compiler-plugin:2.5.1:compile
  </compile>
  <process-test-resources>
    org.apache.maven.plugins:maven-resources-plugin:2.6:testResources
  </process-test-resources>
  <test-compile>
    org.apache.maven.plugins:maven-compiler-plugin:2.5.1:testCompile
  </test-compile>
  <test>
    org.apache.maven.plugins:maven-surefire-plugin:2.12.4:test
  </test>
  <package>
    org.apache.maven.plugins:maven-jar-plugin:2.4:jar
  </package>
  <install>
    org.apache.maven.plugins:maven-install-plugin:2.4:install
  </install>
  <deploy>
    org.apache.maven.plugins:maven-deploy-plugin:2.7:deploy
  </deploy>
</phases>

2. Clean Lifecycle clean lifecycle is defined directly with its plugin bindings.

<phases>
  <phase>pre-clean</phase>
  <phase>clean</phase>
  <phase>post-clean</phase>
</phases>
<default-phases>
  <clean>
    org.apache.maven.plugins:maven-clean-plugin:2.5:clean
  </clean>
</default-phases>

3. Site Lifecycle Site lifecycle is defined directly with its plugin bindings.

<phases>
  <phase>pre-site</phase>
  <phase>site</phase>
  <phase>post-site</phase>
  <phase>site-deploy</phase>
</phases>
<default-phases>
  <site>
    org.apache.maven.plugins:maven-site-plugin:3.3:site
  </site>
  <site-deploy>
    org.apache.maven.plugins:maven-site-plugin:3.3:deploy
  </site-deploy>
</default-phases>

If you want to override these default plugin version you can do it from command prompt as follows

mvn org.apache.maven.plugins:maven-clean-plugin:2.0:clean

instead of

mvn clean:clean