• HTML
• CSS
• JS

HTML

<div class="container">
    <div class="content">
        <h2>Heading...</h2>
        <p>
            Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
            tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
            quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
            consequat.
        </p>
    </div>
    <div class="toggleBtn"></div>
</div>

CSS

@import url('https://fonts.googleapis.com/css?family=Poppins:200,300,400,500,600,700,800,900&display=swap');
*
{
    margin:0;
    padding:0;
    box-sizing:border-box;
    font-family:'Poppins',sans-serif;
}
body
{
    display:flex;
    justify-content:center;
    align-items:center;
    min-height:100vh;
    background:#1a242a;
}
.container
{
    position:relative;
    width:0px;
    height:0px;
    background:#37444b;
    border-radius:25px;
    transition:0.5s;
    display:flex;
    justify-content:center;
    align-items:center;
}
.container.active
{
    width:400px;
    height:400px;
    transition-delay:0.5s;
}
.container::before
{
    content:'';
    position:absolute;
    bottom:-15px;
    width:40px;
    height:40px;
    background:#37444b;
    border-radius:5px;
    opacity:0;
    transition:0.5s;
    transform:rotate(45deg);
}
.container.active::before
{
    opacity:1;
    transition-delay:0.5s;
}
.container .content
{
    min-width:400px;
    padding:40px;
    color:#fff;
    opacity:0;
    transition:0.5s;
    transform:scale(0);
}
.container.active .content
{
    opacity:1;
    transition-delay:0.5s;
    transform:scale(1);
}
.toggleBtn
{
    position:absolute;
    bottom:-20px;
    min-width:60px;
    height:60px;
    background:#0bcf9c;
    border-radius:50%;
    cursor:pointer;
    display:flex;
    justify-content:center;
    align-items:center;
    transition:0.5s;
}
.toggleBtn::before
{
    content:'+';
    font-size:2.5em;
    color:#fff;
}
.container.active .toggleBtn
{
    bottom:-90px;
    transform:rotate(135deg);
    background:#ff5a57;
}

JS

let toggleBtn = document.querySelector('.toggleBtn');
let container = document.querySelector('.container');

toggleBtn.onclick = function(){
    container.classList.toggle('active');
}