Wie gesagt – selbst wenn das Merging von js und css an ist, ist dieses extern eingebundene Script nicht bei den gemergeten Files dabei.
Deshalb wollt ich testen, ob man dieses Script einfach mit einbindet in das Merging. deshalb mein Versuch über das andere Filter es einzubinden.

thanks Dude – but this file is not compressed / merged by default.

what about the trial to force include this file – i tried this:

add_filter('avf_force_include_asset', 'avia_force_include_files', 10, 1);
function avia_force_include_files($force_included) {
	$force_include_js = array('youtube-singleplay');
	$force_included['js'] = array_merge($force_included['js'], $force_include_js);
  return $force_included;
}

but it does not include that little external script (enqueued via child-theme functions.php)

do i have to give in this line the path to the script:
$force_include_js = array('youtube-singleplay');

by the way you can see script in action here ( but with bwp minify): https://webers-testseite.de/youtube-videos/
Password is : Enfold
start one video and than the next without stopping the first.
because i don’t want to style every test-page DSGVO konform (GDPR)

so my observation now is that these files enqueued by child-theme functions.php or some plugins are not included in merging process.
The Script files goes to a footer merged file and this is behind my script. Don’t know why this is important for the script to work.
My trial to include these scripts of the functions.php of my child-theme via :

add_filter('avf_merge_assets', function() {
    return array('css' => 'all', 'js' => 'all');
});

does not work – they were not included.
Edit: bwp-minify does include those files and the script works in this constallation. ( Enfold disable merging via Enfold Options)

Aber selbst dann Günter – parallax bedeutet doch für mich, dass zB Caption und Hintergrund unterschiedliches Scrollverhalten haben sollten.
So wie halt in einer Color-Section auch. Beim Fullscreen bleibt das Caption einfach äquidistant zum Hintergrund und beide scrollen gleich weg.

by the way on this sample it is like on my setup: https://kriesi.at/themes/enfold/homepage/home-v7-one-page-portfolio/

this here should be the expected parallax behavior, except for the fact that only one caption container for three slides can be seen here. So I wonder, how did you create this example? On your own demo page it is like in my example that the scrolling behavior of captions and background is the same. Since the parallax goes on the unordered list, and both background and captions are in it.

thanks for that detailed answer.

oha – much to learn. But you should never give up – not even at an advanced age.
I think the idea of creating a new alb element for that seems to be a good idea. But these are more academic projects than practical necessities.
On practice it is much easier to replace in Enfold Shortcode (debug mode enabled) the image urls.
So when i got time i will try to get familiar more with the youtube api and do the custom alb.
Thanks again

so i got this code for now:
(just for that test page-id: 33150)

function youtube_fix(){
if (is_page(33150)){
?>
<script>
(function($){ 

		$('a.avia_image[href*="www.youtube"]').each(function() {
			regex = /(?:youtube(?:-nocookie)?\.com\/(?:[^\/\n\s]+\/\S+\/|(?:v|e(?:mbed)?)\/|\S*?[?&]v=)|youtu\.be\/)([a-zA-Z0-9_-]{11})/;
			var ID = $(this).attr("href").match(regex)[1];
			$(this).find('img').attr("src", 'https://img.youtube.com/vi/'+ ID +'/maxresdefault.jpg');
		});

	$('body').on('DOMNodeInserted', function(){
		$('iframe.mfp-iframe[src*="youtube.com"]').each(function() {
		var _src = $(this).attr("src")
		$(this).attr("src", _src +  '&cc_load_policy=1&enablejsapi=1&ecver=2&playsinline=1&rel=0&showinfo=0&color=white&iv_load_policy=3')
		});
	})
})(jQuery);
</script>
<?php
}
}
add_action('wp_footer', 'youtube_fix');

but if there was no maxresdefault.jpg it should show the originally used image – how can i insert here an if else ??
see now test-page – the one on the left got a maxres Thumbnail – the right one not –
the 0.jpg always exists – but has black bars top/bottom
https://webers-testseite.de/youtube-videotestseite/