the following example:

create a ScrollToTop button in vanilla - old school style

"use strict";

  document.addEventListener( 'DOMContentLoaded', function () {

    const scrollButton = document
      .createElementNS( 'http://www.w3.org/2000/svg', 'svg' );

      scrollButton.setAttributeNS( 'http://www.w3.org/2000/xmlns/', 
        'xmlns:xlink', 'http://www.w3.org/1999/xlink' );
      scrollButton.setAttribute( 'viewBox', "0 0 100 100" );

      scrollButton.style.width ='50px'
      scrollButton.style.position = 'fixed'
      scrollButton.style.right = '10px'
      scrollButton.style.bottom = '10px'
      scrollButton.style.zIndex = '1000'
      scrollButton.style.opacity = '0.5'
      scrollButton.style.cursor = 'pointer'
      scrollButton.style.transition = 'all ease 400ms'
      scrollButton.style.display = 'none'

    const myCircle = document
      .createElementNS( 'http://www.w3.org/2000/svg', 'circle' );
      myCircle.setAttribute( 'cx', '50' );
      myCircle.setAttribute( 'cy', '50' );
      myCircle.setAttribute( 'r', '50' );
      myCircle.setAttribute( 'fill', 'red' );

    scrollButton.appendChild( myCircle );

    const myArrow = document
      .createElementNS( 'http://www.w3.org/2000/svg', 'path' );

      myArrow.setAttribute( 'd', 'M50 80 L50 20 M50 20 L80 50 M50 20 L20 50' );
      myArrow.setAttribute( 'stroke', 'white' );
      myArrow.setAttribute( 'stroke-width', '15' );
      myArrow.setAttribute( 'stroke-linecap', 'round' );

    scrollButton.appendChild( myArrow );

    scrollButton.addEventListener( 'mouseover', 
      () => scrollButton.style.opacity = '1'
    );

    scrollButton.addEventListener( 'mouseout', 
      () => scrollButton.style.opacity = '.3'
    );

    scrollButton.addEventListener( 'click',  () => document
      .querySelector( 'body' )
      .scrollIntoView( { behavior: 'smooth' } )
    );

    document.body.appendChild( scrollButton );

    window.onscroll = () =>
      scrollButton.style.display =
          ( document.querySelector( 'html' ).scrollTop / 
           ( document.querySelector( 'html' ).scrollHeight - 
            document.querySelector( 'html' ).clientHeight ) ) > 0.25
        ? '' : 'none';

  })

now the same solution using a selector engine.
In this example I used my own engine, it doesn't matter for the comparison.

<script type="text/javascript" src="https://w-web.de/myquery/myquery-min.js"></script>

<script type="text/javascript" src="https://w-web.de/myquery/myquery.js"></script>

"use strict";
$().ready( () => {

    let scrollButton = 
      $().sCreate( '0 0 100 100' )

      .styles()
        .width(  '50px' ).position( 'fixed' )
        .right( '10px' ).bottom( '10px' )
        .zIndex( '1000' ).opacity( '0.3' )
        .cursor( 'pointer' ).transition( 'all ease 400ms' )
        .display( 'none' ).parent()

      .sAppend( 
        $().sCircle( 50, 50, 50 ).fill( 'red' ) )

      .sAppend(
        $().sPath( 'M50 80 L50 20 M50 20 L80 50 M50 20 L20 50' )
          .shortStroke( 'white' , '15', 'round' ) )

      .on( 'mouseover', function (){ this.style.opacity = '1' } )

      .on( 'mouseout', function (){ this.style.opacity = '.3' } )

      .click( () => $( 'body' ).n.scrollIntoView( { behavior: 'smooth' } ) )

      .appendEnd( $( 'body' ) )

    $(window).on('scroll', () => 
        scrollButton.n.style.display =
          ( $('html').n.scrollTop / 
           ( $('html').n.scrollHeight - 
             $('html').n.clientHeight ) ) > 0.25
        ? '' : 'none'
    ) 
  })

Advantages of a SelectorEngine

shorter code
Chaining

Disadvantages

greater learning effort

I think that if you have to deal with a lot of changing employees in a team, the vanilla JS solution certainly makes more sense.

If you are a lone fighter, you should use a Selector Engine.

what is your opinion?

Do you need a selector engine like jQuery, Sizzle or something else?