I think you're overthinking the system. Unfortunately the nomenclature isn't helping. The Thumbnail here is the actual image when you use get_the_post_thumbnail_url().
as an example. Say today you uploaded an image called myimage.jpg.
That would go, if you're storing images by date into this directory:
Now when you call :
$featured_img_url = get_the_post_thumbnail_url('full'); //(or leave the parameter blank)
you would get:
On the other hand if you used this code:
$featured_img_url = get_the_post_thumbnail_url('thumbnail'); //(or choose a different thumbnail size..medium...large...)
it would return:
the autogenerated image based on your thumbnail settings.
to avoid the naming confusion you could also try...
but this requires the actual attachment id, not post id, whether or not you're in the loop.
$imgid = 6; //need to get it dynamically
$imgurldesktop = wp_get_attachment_image_url( $imgid, '' ); //use default image size
$imgurlmobile = wp_get_attachment_image_url( $imgid, 'home-slide-img-mobile' ); //use custom set size