Replace REST API with @wpgraphql with @apollographql client.

learndash-graphql.php

<?php
add_filter( 'register_post_type_args', function( $args, $post_type ) {

if ( 'sfwd-courses' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'course';
	$args['graphql_plural_name'] = 'courses';
}

if ( 'sfwd-lessons' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'lesson';
	$args['graphql_plural_name'] = 'lessons';
}

if ( 'sfwd-quiz' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'quiz';
	$args['graphql_plural_name'] = 'quizs';
}
if ( 'sfwd-topic' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'topic';
	$args['graphql_plural_name'] = 'topics';
}

return $args;

}, 10, 2 );


add_filter( 'register_taxonomy_args', function( $args, $taxonomy ) {

if ( 'doc_tag' === $taxonomy ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'documentTag';
	$args['graphql_plural_name'] = 'documentTags';
}

return $args;

}, 10, 2 );


add_action( 'graphql_register_types', function() {
register_graphql_field( 'Courses', 'featured_media', [
	 'type' => 'String',
	 'description' => __( 'The featured_media of the course', 'learndash' ),
	 'resolve' => function( $post ) {
		 $featured_media = get_post_meta( $post->ID, 'featured_media', true );
		 return ! empty( $featured_media ) ? $featured_media : 'null';
	 }
] );
} );


add_action( 'graphql_register_types', function() {

$post_types = WPGraphQL::$allowed_post_types;

if ( ! empty( $post_types ) && is_array( $post_types ) ) {
	foreach ( $post_types as $post_type ) {
		$post_type_object = get_post_type_object( $post_type );

		if ($post_type === 'sfwd-courses') {
			register_graphql_field( $post_type_object->graphql_single_name, 'blurb', [
				'type' => 'String',
				'description' => __( 'The blurb of the course', 'sfwd-courses' ),
				'resolve' => function( $post ) {
					$blurb = get_post_meta( $post->ID, 'blurb', true );
					return ! empty( $blurb ) ? $blurb : '';
				}
			]);
		}

		if ($post_type === 'sfwd-quiz') {
			register_graphql_field( $post_type_object->graphql_single_name, 'total', [
				'type' => 'String',
				'description' => __( 'The total of the quiz', 'sfwd-quiz' ),
				'resolve' => function( $post ) {
					$price = get_post_meta( $post->ID, 'total', true );
					return ! empty( $price ) ? $price : '';
				}
			]);
		}
	}
}
});

I had to add this code below the code above to get all my associated topics of a lesson, and lessons to a course:

add_action( 'graphql_register_types', function() {

    register_graphql_connection( [
        'fromType' => 'Course',
        'toType' => 'Lesson',
        'fromFieldName' => 'siblings',
        'resolve' => function( $page, $args, $context, $info ) {
            $courseId = $page->databaseId;
            
            if ( ! $courseId ) {
                return null;
            }

            $resolver = new \WPGraphQL\Data\Connection\PostObjectConnectionResolver( $page, $args, $context, $info );
            $resolver->set_query_arg( 'post_type', 'sfwd-lessons' ); // Assuming 'sfwd-lessons' is your Lesson post type
            $resolver->set_query_arg( 'meta_query', array(
                array(
                    'key' => 'course_id',
                    'value' => $courseId,
                    'compare' => '=',
                ),
            ));

            return $resolver->get_connection();
        }
    ]);
	
	register_graphql_connection( [
        'fromType' => 'Lesson',
        'toType' => 'Topic',
        'fromFieldName' => 'siblings',
        'resolve' => function( $page, $args, $context, $info ) {
            $lessonId = $page->databaseId;
            
            if ( ! $lessonId ) {
                return null;
            }

            $resolver = new \WPGraphQL\Data\Connection\PostObjectConnectionResolver( $page, $args, $context, $info );
			$resolver->set_query_arg( 'post_type', 'sfwd-topic' ); // Assuming 'sfwd-topic' is your Lesson post type
      
            $resolver->set_query_arg( 'meta_query', array(
                array(
                    'key' => 'lesson_id',
                    'value' => $lessonId,
                    'compare' => '=',
                ),
            ));

            return $resolver->get_connection();
        }
    ]);

});

query CoursesAndLessons {
  courses {
    nodes {
      title
      siblings {
        nodes {
          title
          siblings {
            nodes {
              title
            }
          }
        }
      }
    }
  }
}

returns:

{
  "data": {
    "courses": {
      "nodes": [
        {
          "title": "Course 2",
          "siblings": {
            "nodes": [
              {
                "title": "C2 – Lesson 2",
                "siblings": {
                  "nodes": [
                    {
                      "title": "C2 – L2- T2"
                    },
                    {
                      "title": "C2 – L2- T1"
                    }
                  ]
                }
              },
              {
                "title": "C2 – Lesson 1",
                "siblings": {
                  "nodes": []
                }
              }
            ]
          }
        },
        {
          "title": "Course 1",
          "siblings": {
            "nodes": [
              {
                "title": "Lesson 3",
                "siblings": {
                  "nodes": []
                }
              },
              {
                "title": "Lesson 2",
                "siblings": {
                  "nodes": []
                }
              },
              {
                "title": "Lesson 1",
                "siblings": {
                  "nodes": []
                }
              }
            ]
          }
        }
      ]
    }
  }
}