SOURCE

<header>header</header>
<div class="container">
<div class="aside">aside</div>
<div class="section">section</div>
</div>
<footer>footer</footer>

*{
    margin: 0;
    padding: 0;
    width: 100%;
  }
  header{
    height: 100px;
    background-color: red;
  }
  footer{
    position: fixed;
    bottom:0;
    width: 100%;
    height: 100px;
    background-color: yellow;
  }
  
  @media screen and (min-width:500px) {
    .container{
      position: fixed;
      top:110px;
      bottom: 110px;
      display: flex;
    }
    .aside{
      width: 200px;
      height: 200px;
      background-color: blue;
      margin-right: 10px;
    }
    .section{
      flex: 1;
      align-self: stretch;
      background-color: green;
    }
  }

    @media screen and (max-width:500px) {
    .container{
      position: fixed;
      top:110px;
      bottom: 110px;
      display: flex;
      flex-direction: column;
    }
    .aside{
      width: 100%;
      height: 200px;
      background-color: blue;
      margin-bottom: 10px;
    }
    .section{
      flex: 1;
      align-self: stretch;
      background-color: green;
    }
  }