Plugin series

series.php

<?php
/*
Plugin Name: Nested Route
*/

add_action('init', 'rpt');
function rpt() {
    // flush_rewrite_rules();
    register_post_type('series', [
        'label' => 'series',
        'public' => true,
        'supports' => ['title', 'editor'],
        'rewrite' => [
            'with_front' => false,
            'slug' => '/'
        ]
    ]);
    register_post_type('episode', [
        'label' => 'episode',
        'public' => true,
        'supports' => ['title', 'editor']
    ]);
    add_rewrite_rule("series/(.*?)/episodes/(.*?)[/]?$", 'index.php?series=$matches[1]&episode=$matches[2]', 'top');
}

add_action('add_meta_boxes', 'metabox_series');
function metabox_series() {
    add_meta_box('series', 'Series', 'series_cb', 'episode');
}
add_action('save_post', 'save_episode');
function save_episode($post) {
    if(@$_POST['series_id']) {
        update_post_meta($post, 'series_id', $_POST['series_id']);
    }
}
function series_cb($post) {
    $q = new WP_Query([
        'posts_per_page' => -1,
        'post_type' => 'series'
    ]);
    $series_id = get_post_meta($post->ID, 'series_id', true);
    ?>
    <label for="series_id">Choose Series</label>
    <select name="series_id" id="series_id">
    <?php
    while($q->have_posts()): $q->the_post();
    $checked = $series_id && $series_id !== '' && $series_id == get_the_ID();
    ?>
        <option value="<?= get_the_ID() ?>" <?= $checked ? 'selected' : '' ?>><?= get_the_title() ?></option>
    <?php endwhile ?>
    </select>
    <?php 
}
add_action('post_type_link', 'episode_link', 10, 2);
function episode_link($link, $post) {
    if($post->post_type == 'episode') {
        $meta = get_post_meta($post->ID, 'series_id', true);
        if(!$meta || $meta == "") return $link;
        $series = get_post($meta);
        return home_url("/series/{$series->post_name}/episodes/{$post->post_name}");
    }
    return $link;
}
?>