For Creating custom api in wordpress we need to register a new route url for our custom rest api.

Go the theme functions.php file and add the code

add_action('rest_api_init', function() {
	register_rest_route('twentyone_wl/v1', 'posts', [
		'methods' => 'GET',
		'callback' => 'twentyone_wl_posts',
	]);
    

});
/*'twentyone_wl/v1' is namespace define*/

Now add the function which gives some data when you hit the custom api url

function twentyone_wl_posts() {
	$args = [
		'numberposts' => 99999,
		'post_type' => 'post'
	];

	$posts = get_posts($args);

	$data = [];
	$i = 0;

	foreach($posts as $post) {
		$data[$i]['id'] = $post->ID;
		$data[$i]['title'] = $post->post_title;
		$data[$i]['content'] = $post->post_content;
		$data[$i]['slug'] = $post->post_name;
		$data[$i]['featured_image']['thumbnail'] = get_the_post_thumbnail_url($post->ID, 'thumbnail');
		$data[$i]['featured_image']['medium'] = get_the_post_thumbnail_url($post->ID, 'medium');
		$data[$i]['featured_image']['large'] = get_the_post_thumbnail_url($post->ID, 'large');
		$i++;
	}

	return $data;
}

Now hit the url in browser for Ex: http://localhost/demowordpress/wp-json/twentyone_wl/v1/posts

Now You see this type JSON data for your custom api

Thanks for visting and comment below if it is useful for You!!!!