How to create a custom endpoint using Drupal JSON:API

composer require drupal/jsonapi_resources

# Defines a route for a collection containing featured articles nodes.
my_custom_resource.jsonapi_resources.featured_articles:
  # %jsonapi% is a placeholder for the JSON:API base path, which can be
  # configured in a site's services.yml file.
  path: '/%jsonapi%/articles/featured'
  defaults:
    # Every JSON:API resource route must declare a _jsonapi_resource. The
    # value can either be a class or a service ID. Unlike the _controller
    # route default, it is not possible to declare a method name to be called.
    _jsonapi_resource: Drupal\my_custom_resource\Resource\FeaturedArticlesResource
    _jsonapi_resource_types: ['node--article']
  requirements:
    _permission: 'access content'

# Defines a route for a collection containing related articles of given node.
  path: '/%jsonapi%/articles/{page}/related'
  defaults:
    _jsonapi_resource: Drupal\my_custom_resource\Resource\RelatedArticlesOfPageResource
    _jsonapi_resource_types: ['node--article']
  requirements:
    _permission: 'access content'
  options:
    parameters:
      agent:
        type: entity:node

<?php

namespace Drupal\my_custom_resource\Resource;

use Drupal\Core\Cache\CacheableMetadata;
use Drupal\jsonapi\ResourceResponse;
use Drupal\jsonapi_resources\Resource\EntityQueryResourceBase;
use Drupal\node\NodeInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * Processes a request for a collection containing featured articles nodes.
 */
class FeaturedArticlesResource extends EntityQueryResourceBase {

  /**
   * Process the resource request.
   *
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The request.
   *
   * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
   * @throws \Drupal\Component\Plugin\Exception\PluginNotFoundException
   *
   * @return \Drupal\jsonapi\ResourceResponse
   *   The response.
   */
  public function process(Request $request): ResourceResponse {
    $cacheability = new CacheableMetadata();

    /** @var \Drupal\Core\Entity\ContentEntityType $entity_type */
    $entity_type = $this->entityTypeManager->getDefinition('node');
    /** @var string $bundle_field */
    $bundle_field = $entity_type->getKey('bundle');
    /** @var string $status_field */
    $status_field = $entity_type->getKey('status');

    $entity_query = $this->getEntityQuery('node')
      ->condition($bundle_field, 'article')
      ->condition('promote', TRUE)
      ->condition($status_field, NodeInterface::PUBLISHED);

    $cacheability->addCacheContexts(['url']);

    $paginator = $this->getPaginatorForRequest($request);
    $paginator->applyToQuery($entity_query, $cacheability);

    $data = $this->loadResourceObjectDataFromEntityQuery($entity_query, $cacheability);

    $pagination_links = $paginator->getPaginationLinks($entity_query, $cacheability, TRUE);

    /** @var \Drupal\jsonapi\CacheableResourceResponse $response */
    $response = $this->createJsonapiResponse($data, $request, 200, [], $pagination_links);
    $response->addCacheableDependency($cacheability);

    return $response;
  }

}

# Defines a route for a collection containing related articles of given node.
  path: '/%jsonapi%/articles/{page}/related'
  defaults:
    _jsonapi_resource: Drupal\my_custom_resource\Resource\RelatedArticlesOfPageResource
    _jsonapi_resource_types: ['node--article']
  requirements:
    _permission: 'access content'
  options:
    parameters:
      agent:
        type: entity:node
        # Upcast entity parameters by UUID.
        converter: 'paramconverter.jsonapi.entity_uuid'