SCSS

HTML

<a href="#" class="header">
  THIS IS THE HEADER 
  <span style="font-size: 12px;">(you can hover me)</span><br>
  <span style="font-size: 14px; font-style: italic; letter-spacing: 0.05em;">which should be <br> OVER "invisible section" AND "fixed section" <br> BUT UNDER "first section", <br>WHILE maintaining fixed position</span>
</a>
<div class="section section-fixed">
  fixed section underneath "invisible section"
</div>
<div class="wrapper">
  <div class="section section-invisible">
    <h2>invisible section</h2>
  </div>
  <div class="section section-first">
    first section
  </div>
</div>

SCSS

@import url('https://fonts.googleapis.com/css2?family=Baloo+Da+2&display=swap');

html {
  box-sizing: border-box;
}

*, *:before, *:after {
  box-sizing: inherit;
}

html, body {
  margin: 0;
  padding: 0;
}

body {
  font-size: 16px;
  font-family: 'Baloo Da 2', cursive;
}

.header {
  background: floralwhite;
  display: inline-block;
  padding: 10px;
  position: fixed;
  z-index: 3;
  left: 30px;
  top: 30px;
  color: inherit;
  text-decoration: none;
  line-height: 1.2;
  box-shadow: 0 0 0 rgba(0,0,0,.5);
  transition: all .5s ease;

  &:hover {
    box-shadow: 2px 3px 10px rgba(0,0,0,.5);
    background-color: pink;
  }
}

.wrapper {
  border: 2px solid red;
  z-index: 2;
  position: relative;
}

.section {
  padding: 20px;
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;

  &-fixed {
    background: gray;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    z-index: 1;
  }

  &-invisible {
    align-items: flex-end;
    padding: 0;
   
   h2 {
     background: goldenrod;
     padding: 50px;
     margin: 0;
     width: 100%;
     text-align: center;
   }
  }
  
  &-first {
    background: darkslateblue;
    color: white;
  }
}