You can do this as follows:
function mostraDiv(num) {
document.getElementById('div1').style.display='none';
document.getElementById('div2').style.display='none';
document.getElementById('div3').style.display='none';
document.getElementById('div'+num).style.display='block';
}
.hiddenEl{display:none;}
<div class="sidebarContainer">
<button class="imagens" onclick="mostraDiv('1')">Mostra Div 1</button>
<button class="imagens" onclick="mostraDiv('2')">Mostra Div 2</button>
<button class="imagens" onclick="mostraDiv('3')">Mostra Div 3</button>
</div>
<div class="mainContainer">
<div id="div1">Sou a div1</div>
<div id="div2" class="hiddenEl">Sou a div2</div>
<div id="div3" class="hiddenEl">Sou a div3</div>
</div>
Then just make the modifications to your measure, changing the buttons by divs
and inserting images inside them as preview thumbnails of the video that the user will see, clicking on the desired image / button and also popular% content% (titles, videos, etc.).
I've created here an example in JSFiddle simple but well thought out how this will work with the respective titles and videos for each button, to get an idea.