https://www.maxlaumeister.com/blog/hide-related-videos-in-youtube-embeds/ but readable

hide-youtube-related.html

<div class="hytPlayerWrapOuter">
  <div class="hytPlayerWrap">
    <iframe width="640" height="360" src="https://www.youtube.com/embed/0sDg2h3M1RE?rel=0&enablejsapi=1" frameborder="0" ></iframe>
  </div>
</div>

hide-youtube-related.js

document.addEventListener( 'DOMContentLoaded', function() { 
    
    if ( window.hideYTActivated ) return; 
    
    let onYouTubeIframeAPIReadyCallbacks=[]; 
    
    for ( let playerWrap of document.querySelectorAll( ".hytPlayerWrap" ) ) { 
        
        let playerFrame = playerWrap.querySelector("iframe");
        
        let tag = document.createElement( 'script' );
        tag.src="https://www.youtube.com/iframe_api"; 

        let firstScriptTag = document.getElementsByTagName( 'script' )[0];
        firstScriptTag.parentNode.insertBefore( tag, firstScriptTag );
        
        let onPlayerStateChange = function( event ) { 
            
            if ( event.data == YT.PlayerState.ENDED ) {
                
                playerWrap.classList.add( "ended" );
            
            }
            else if ( event.data == YT.PlayerState.PAUSED ) {
                
                playerWrap.classList.add( "paused" );
            
            }
            else if ( event.data == YT.PlayerState.PLAYING ) {
                
                playerWrap.classList.remove( "ended" ); 
                playerWrap.classList.remove( "paused" ); 

            }
        
        }; 
        
        let player; 
        
        onYouTubeIframeAPIReadyCallbacks.push( function() { 
            
            player = new YT.Player( playerFrame, {
                events: {
                    'onStateChange': onPlayerStateChange
                }
            } );
        
        } );
        
        playerWrap.addEventListener( "click", function() { 
            
            let playerState = player.getPlayerState();
            
            if ( playerState == YT.PlayerState.ENDED ) { 
                
                player.seekTo( 0 );
            
            }
            else if ( playerState == YT.PlayerState.PAUSED ) {
                
                player.playVideo();
            
            }
        
        } );
    
    }
    
    window.onYouTubeIframeAPIReady = function() {
        
        for ( let callback of onYouTubeIframeAPIReadyCallbacks ) {
            callback();
        }

    };
    
    window.hideYTActivated = true;

} );

hide-youtube-related.scss

.hytPlayerWrap {
    
    display: inline-block;
    position: relative;

    &.ended {
        
        &:after {

            content: ""; 
            position: absolute; 
            top: 0; 
            left: 0; 
            bottom: 0; 
            right: 0; 
            cursor: pointer; 
            background-color: #000; 
            background-repeat: no-repeat; 
            background-position: center;
            background-size: 64px 64px;
            background-image: url( data:image/svg+xml;utf8;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxMjgiIGhlaWdodD0iMTI4IiB2aWV3Qm94PSIwIDAgNTEwIDUxMCI+PHBhdGggZD0iTTI1NSAxMDJWMEwxMjcuNSAxMjcuNSAyNTUgMjU1VjE1M2M4NC4xNSAwIDE1MyA2OC44NSAxNTMgMTUzcy02OC44NSAxNTMtMTUzIDE1My0xNTMtNjguODUtMTUzLTE1M0g1MWMwIDExMi4yIDkxLjggMjA0IDIwNCAyMDRzMjA0LTkxLjggMjA0LTIwNC05MS44LTIwNC0yMDQtMjA0eiIgZmlsbD0iI0ZGRiIvPjwvc3ZnPg== );

        }

    }

    &.paused {

        &:after {

            content: "";
            position: absolute;
            top: 70px;
            left: 0;
            bottom: 50px;
            right: 0;
            cursor: pointer;
            background-color: #000;
            background-repeat: no-repeat;
            background-position: center;
            background-size: 40px 40px;
            background-image: url( data:image/svg+xml;utf8;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZlcnNpb249IjEiIHdpZHRoPSIxNzA2LjY2NyIgaGVpZ2h0PSIxNzA2LjY2NyIgdmlld0JveD0iMCAwIDEyODAgMTI4MCI+PHBhdGggZD0iTTE1Ny42MzUgMi45ODRMMTI2MC45NzkgNjQwIDE1Ny42MzUgMTI3Ny4wMTZ6IiBmaWxsPSIjZmZmIi8+PC9zdmc+ );

        }

    }

}