private static final ModelType<ProjectIdentifier> PROJECT_IDENTIFIER_MODEL_TYPE = ModelType.of(ProjectIdentifier.class);
    private static final ModelType<ExtensionContainer> EXTENSION_CONTAINER_MODEL_TYPE = ModelType.of(ExtensionContainer.class);
    private static final Logger BUILD_LOGGER = Logging.getLogger(Project.class);

    private final ProjectState owner;
    private final ClassLoaderScope classLoaderScope;

    // compile and runtime classpaths for version mapping, even if a non-main feature is being published.
    private static void configurePublishing(PluginContainer plugins, ExtensionContainer extensions, SourceSet sourceSet) {
        plugins.withType(PublishingPlugin.class, plugin -> {
            PublishingExtension publishing = extensions.getByType(PublishingExtension.class);

                }

                static class Rules extends RuleSource {
                    @Model
                    MyExtension myExtension(ExtensionContainer extensions) {
                        extensions.getByType(MyExtension)
                    }

                    @Model
                    String value(MyExtension myExtension) {

import org.gradle.api.internal.artifacts.transform.UnzipTransform;
import org.gradle.api.internal.project.ProjectInternal;
import org.gradle.api.model.ObjectFactory;
import org.gradle.api.plugins.ExtensionContainer;
import org.gradle.api.provider.Provider;
import org.gradle.api.publish.PublishingExtension;
import org.gradle.api.publish.maven.MavenPublication;

A plugin can add properties and methods to a project using extensions.

The link:{groovyDslPath}/org.gradle.api.Project.html[`Project`] object has an associated link:{javadocPath}/org/gradle/api/plugins/ExtensionContainer.html[`ExtensionContainer`] object that contains all the settings and properties for the plugins that have been applied to the project.

                        "getExtensions",
                        "()Lorg/gradle/api/plugins/ExtensionContainer;"
                    )
                    LDC(name.original)
                    ALOAD(1)
                    INVOKEINTERFACE(
                        GradleTypeName.extensionContainer,
                        "configure",
                        "(Ljava/lang/String;Lorg/gradle/api/Action;)V"

import org.gradle.api.internal.tasks.properties.ServiceReferenceSpec;
import org.gradle.api.logging.Logger;
import org.gradle.api.logging.Logging;
import org.gradle.api.plugins.ExtensionContainer;
import org.gradle.api.provider.Property;
import org.gradle.api.provider.Provider;
import org.gradle.api.services.BuildService;
import org.gradle.api.services.internal.BuildServiceProvider;

import org.gradle.api.internal.provider.DefaultProviderFactory;
import org.gradle.api.model.ObjectFactory;
import org.gradle.api.plugins.Convention;
import org.gradle.api.plugins.ExtensionAware;
import org.gradle.api.plugins.ExtensionContainer;
import org.gradle.api.provider.ListProperty;
import org.gradle.api.provider.MapProperty;
import org.gradle.api.provider.Property;
import org.gradle.api.provider.Provider;
import org.gradle.api.provider.SetProperty;

import org.gradle.api.internal.provider.support.LazyGroovySupport;
import org.gradle.api.invocation.Gradle;
import org.gradle.api.plugins.ExtensionAware;
import org.gradle.api.plugins.ExtensionContainer;
import org.gradle.api.services.ServiceReference;
import org.gradle.cache.internal.CrossBuildInMemoryCache;
import org.gradle.cache.internal.CrossBuildInMemoryCacheFactory;
import org.gradle.internal.DisplayName;

Plugins do this using *extension objects*.

A link:{groovyDslPath}/org.gradle.api.Project.html[Project] has an associated link:{javadocPath}/org/gradle/api/plugins/ExtensionContainer.html[ExtensionContainer] object that contains all the settings and properties for the plugins that have been applied to the project.
You can provide configuration for your plugin by adding an extension object to this container.