well jQuery 3.5.1 got some changes and if your snippet contains:

$(window).load(function(){

just replace that line with:

$(window).on('load', function(){

…
please check if on youtube – the link works – when you go to share – and press embed.
If the video has no permittance to be “embedable” you will see this – allthough the video plays on youtube ( see darkend Background)

your video is playing here on my installation:
https://consulting.webers-testseite.de/bonn/

Where is the link ? (Anyway i got it in my browserhistory )

You have updated to WordPress 5.8 – maybe there are some inconsistencies.

hint for: https://kriesi.at/support/topic/problem-with-cookie-modal/#post-1212640
you can solve the samesite cookie message by adding this to your wp-config.php:

define( 'WP_SAMESITE_COOKIE', 'Lax' );
you can try strict or none – but for me lax is enough to get rid of that.
and here is a good place for it:

Your answer is probably purposeful.
It would be helpful to us normal mortals an update resistant solution which is presented to us. With hints here the fewest can begin something.

Sorry didn’t find any other solution – than to check if the last string has 11 characters. All youtube IDs got 11characters.

function insert_video_by_description(){
if(is_archive()){
?>
<script>
(function($){
$(document).ready(function(){
var my_portfolio = $('.archive.tax-portfolio_entries');
	$(my_portfolio).find('p').contents().filter(function(){
	return this.nodeType === 3;
	}).wrap('<span/>');
	var youtube_video = $(my_portfolio).find('.category-term-description').find('p:last-child').find('span:last-child').text().trim();	
	if(youtube_video.length === 11){
		$(my_portfolio).find('.category-term-description').find('p:last-child').find('span:last-child').css('display', 'none');
		$(my_portfolio).find('.category-term-description').append('<div class="product-video"><iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/'+youtube_video+'?autoplay=1" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></div>');
	};
});
})(jQuery);
</script>
<?php
}
}
add_action('wp_footer', 'insert_video_by_description');

This to your quick css:

.category-term-description {
    position: relative;
    max-width: 600px
}

.category-term-description .product-video {
    position: relative;
    padding-bottom: 56.25%;
    padding-top: 0;
    height: 0;
    overflow: hidden;
}

.product-video iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}

…

See solution here: Link

smarter solution to leave the client the possibility to do it himself.

No-Chance on that.

try that link on your button anyway. Even in advanced layer slider ( see example page above ) that “trick” will do the job.

And you do not like to link to a youtube video in a lightbox?
Example Link

What do you mean by: “It should not show the youtube video only when the linked is clicked.”

sorry wrong posting

if your settings in Enfold Options on layout dimensions do not work correct – there could be a synthax error in your quick css.
These settings including custom.css are placed on the bottom of all other css:

.container {
 width:100%
}
.container .av-content-small.units {
 width:75%
}
.responsive .boxed#top ,
.responsive.html_boxed.html_header_sticky #header,
.responsive.html_boxed.html_header_transparency #header {
 width:1310px;
 max-width:90%
}
.responsive .container {
 max-width:1310px
}
#top #header .av-main-nav > li > a {
 font-size:13px;
 font-weight:normal
}
@media only screen and (min-width:768px) {
}
@media only screen and (max-width:767px) {
}

so if you got f.e. a missing closing bracket or instead of a semicolon you set a comma – sometimes all the rest of the following css will not work.

you can post your quick css content here (but please use the code tag) so we can see if my guess is relevant