example.css

* {
  margin: 0;
  padding: 0;
}

body {
  display: flex;
  min-height: 100vh;
  flex-direction: column;
  justify-content: space-between;
}

.header {
  height: 100px;
  background: #eee;
}

.footer {
  height: 100px;
  background: #ccc;
}

.section {
  display: flex;
}

.sidebar {
  width: 25%;
  background: red;
  min-height: calc(100vh - 200px);
}

.body {
  width: 75%;
  background: yellow;
  min-height: calc(100vh - 200px);
}

<!-- <header class="header"></header>
  <section class="section">
      <aside class="sidebar">sidebar</aside>
      <main class="body">body</main>
  </section>
  <footer class="footer"></footer> -->