One of the easiest ways to add a thumbnail image from your YouTube video as a featured image is to to install a plugin.
Video Thumbnails is one of the best plugins for this purpose.
I have tested this plugin locally and it works very well and even supports Custom Posts Types.
You can use this plugin to add thumbnail images to all your archive pages and portfolio style pages as well.
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In my experience the size of the YouTube thumbnail doesn’t fit the featured image size on most themes but if you have a theme where the size works, this is perfect.
That’s right and you can also set a default image in the plugins folder