CongBom

To get meta value in front-end, use <code>get_post_meta()</code> with meta key is <code>youtube_id</code>.
Example:
Print YouTube video ID: 
 ID, ‘youtube_id’, true);?>
Thumbnails: 
 <img src="http://img.youtube.com/vi/ID, ‘youtube_id’, true);?>/mqdefault.jpg” alt=”” />
Video player:
<iframe src="http://www.youtube.com/embed/ID, ‘youtube_id’, true); ?>” width=”588″ height=”331″ frameborder=”0″ allowfullscreen>

YouTube metabox

Create YouTube meta-box for post with live preview

yoren

Yoren Chang

Try it live in your browser — <a href="https://playground.wordpress.net/?blueprint-url=https://raw.githubusercontent.com/1fixdotio/categories-metabox-enhanced/master/blueprint/playground.blueprint.json" rel="nofollow ugc">launch the WordPress Playground demo</a>. Boots a WordPress instance with this plugin active, the Category taxonomy pre-configured as radio buttons, and seeded posts you can edit in the Block Editor or run through Quick Edit / Bulk Edit immediately.
Thanks to <a href="https://github.com/WebDevStudios/Taxonomy_Single_Term" rel="nofollow ugc">Taxonomy_Single_Term</a>, a library created by <a href="http://webdevstudios.com/" rel="nofollow ugc">WebDevStudios</a>, it made my work much easier on creating this plugin.
With Categories Metabox Enhanced, you can:
<ul>
<li>Change the built-in Categories metabox/panel to a single term UI, which means replacing the checkboxes with radio buttons or a select drop-down.</li>
<li>Apply the single term UI to other hierarchical taxonomies from the plugin’s Settings page.</li>
<li>Configure each hierarchical taxonomy independently — the same install can keep one taxonomy as checkboxes while another is forced to radio.</li>
<li>Customize the single term UI by setting these options:
<ul>
<li>Option type (checkbox, radio, or select).</li>
<li>Priority and position (context) of the metabox placement. Classic editor only.</li>
<li>Title of the metabox / Block Editor panel.</li>
<li>Whether child-terms should be indented.</li>
<li>Whether adding new terms from the metabox is enabled.</li>
<li>Whether a term selection is required (Force selection). When on, the classic metabox hides the “None” option, the Block Editor sidebar drops the “— Select —” entry once a term is chosen, and an empty save is substituted with a default term server-side so REST and programmatic callers can’t bypass it.</li>
</ul>
</li>
</ul>
The single-term invariant is enforced everywhere posts can be saved: the Classic Editor metabox, the Block Editor sidebar panel, Quick Edit, Bulk Edit, the REST API, WP-CLI, and any code that calls <code>wp_set_object_terms()</code> directly. The classic-editor and Block Editor surfaces enforce in the UI; everything else is enforced on the server via the <code>set_object_terms</code> action, which coerces multi-term submissions to the last term and substitutes the default term on empty submissions when Force selection is on.
The substituted default term resolves in this order: the <code>default_<taxonomy></code> option (e.g. <code>default_category</code>) → the <code>default_term_<taxonomy></code> option populated by <code>register_taxonomy()</code> <code>default_term</code> arg (WP 5.5+) → the first term ordered by name. Override per taxonomy with the <code>of_cme_force_selection_default_term</code> filter.
The Block Editor sidebar panel is rendered for taxonomies registered with <code>show_in_rest = true</code>. Hierarchical taxonomies that opt out of REST fall back to the Classic Editor metabox path.

Categories Metabox Enhanced

Replace the checkboxes with radio buttons or a select drop-down in the built-in Categories metabox and the Block Editor sidebar panel.

YouTube metabox vs Categories Metabox Enhanced