Get Youtube video thumbnail from Youtube URL in Shopify Liquid

youtube-video-thumbnail.liquid

{% comment %} We assume that we have the youtube video in form of URL {% endcomment %}
{% assign video_url = 'https://www.youtube.com/watch?v=ScMzIvxBSi4' %}
{% assign thumbnail_url = '' %}

{% comment %} First of all, we get last part of the URL that's supposedly the Youtube Video ID {% endcomment %}
{% assign video_id = video_url | split: '/' | last %}

{% comment %} but we need strip any extra URL params {% endcomment %}
{% assign video_id = video_id | split: '?' | first %}

{% comment %} Finally, remove any hash string {% endcomment %}
{% assign video_id = video_id | split: '#' | first %}

{% comment %} It's always good to add additional validations {% endcomment %}
{% if video_id != blank %}
    {% assign thumbnail_url = 'http://img.youtube.com/vi/' | append: video_id | append: '/maxresdefault.jpg' %}
{% endif %}

{{ thumbnail_url }}
{% comment %}
Output:
ScMzIvxBSi4
{% endcomment %}