Check out example codes for "how to do parallax scrolling". It will help you in understanding the concepts better.

Code Example 1

//WORKING EXAMPLE

<main class="wrapper">
  <section class="section parallax bg1">
    <h1>Such Adorableness</h1>
  </section>
  <section class="section static">
    <h1>Boring</h1>
  </section>
  <section class="section parallax bg2">
    <h1>SO FWUFFY AWWW</h1>
  </section>
</main>


.wrapper {
  /* The height needs to be set to a fixed value for the effect to work.
   * 100vh is the full height of the viewport. */
  height: 100vh;
  /* The scaling of the images would add a horizontal scrollbar, so disable x overflow. */
  overflow-x: hidden;
  /* Enable scrolling on the page. */
  overflow-y: auto;
  /* Set the perspective to 2px. This is essentailly the simulated distance from the viewport to transformed objects.*/
  perspective: 2px;
}

.section {
  /* Needed for children to be absolutely positioned relative to the parent. */
  position: relative;
  /* The height of the container. Must be set, but it doesn't really matter what the value is. */
  height: 100vh;

  /* For text formatting. */
  display: flex;
  align-items: center;
  justify-content: center;
  color: white;
  text-shadow: 0 0 5px #000;
}

.parallax::after {
  /* Display and position the pseudo-element */
  content: " ";
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;

  /* Move the pseudo-element back away from the camera,
   * then scale it back up to fill the viewport.
   * Because the pseudo-element is further away, it appears to move more slowly, like in real life. */
  transform: translateZ(-1px) scale(1.5);
  /* Force the background image to fill the whole element. */
  background-size: 100%;
  /* Keep the image from overlapping sibling elements. */
  z-index: -1;
}

/* The styling for the static div. */
.static {
  background: red;
}

/* Sets the actual background images to adorable kitties. This part is crucial. */
.bg1::after {
  background-image: url('https://placekitten.com/g/900/700');
}

.bg2::after {
  background-image: url('https://placekitten.com/g/800/600');
}

Here’s a breakdown of what each class is for:

.wrapper - Sets the perspective and scroll properties for the whole page.
.section - Size, display, and text properties. Mostly not relevant to the parallax effect.
.static - Adds a background to a section, just for demonstration.
.parallax - Adds an ::after pseudo-element with the background image and transforms needed for the parallax effect.
.bg1, .bg2 - Adds the respective background images for each section. (You could use an img tag instead.)

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