plugin of a plugin

[Delany]

Hi. Two things look odd here

        <plugin>
          <groupId>com.github.spotbugs</groupId>
          <artifactId>spotbugs-maven-plugin</artifactId>
          <version>4.7.3.4</version>
          <configuration>
            <plugins>
              <plugin>
                <groupId>com.h3xstream.findsecbugs</groupId>
                <artifactId>findsecbugs-plugin</artifactId>
                <version>1.12.0</version>
              </plugin>
            </plugins>
          </configuration>

Whats the difference between declaring plugins dependencies and declaring
plugin plugins?
Only dependencies are mentioned here:
https://maven.apache.org/guides/mini/guide-configuring-plugins.html

And why is findsecbugs-plugin-1.12.0.jar copied to
${project.build.directory} when I run mvn spotbugs:check?

Kind regards,
Delany