How can i make fancy corners in my website

Question

I want to make corner like in this image

I am a beginner to frontend development. I tried this but i'm stuck now.

The Code:

* {
  margin: 0;
  padding: 0;
}

body {
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
}

.container {
  background-color: white;
  border-radius: 15px
}

.box {
  text-align: center;
  font-size: 50pt;
}

#top {
  background-color: blue;
  position: relative;
  border-bottom-left-radius: 50px
}

#top:after {
  content: "";
  position: absolute;
  right: 0;
  top: 100%;
  height: 100px;
  width: 100px;
  border-top-right-radius: 50%;
  box-shadow: 0 -50px 0 0 black;
}

#bottom {
  background-color: red;
  position: relative;
}

<div class="container">
  <div class="box" id="top">Top</div>
  <div class="box" id="bottom">Bottom</div>
</div>

This HTML and CSS code defines a webpage with a centered container containing two boxes, one with a blue background ("Top") and a decorative shadow, and the other with a red background ("Bottom"). The styling includes rounded corners and a centered text.

Answer 1

I would just add additional containers around the top and bottom ones to set the opposite background-color

* {
  margin: 0;
  padding: 0;
}

body {
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
}

.container {
  background-color: white;
  border-radius: 15px
}

.box {
  text-align: center;
  font-size: 50pt;
}

#OuterTop {
  background-color: red;
}

#OuterBottom {
  background-color: blue;
}

#top {
  background-color: blue;
  position: relative;
  border-bottom-left-radius: 50px;
}

#bottom {
  background-color: red;
  border-top-right-radius: 50px;
  position: relative;
}

<div class="container">
  <div id="OuterTop">
    <div class="box" id="top">Top</div>
   </div>
  <div id="OuterBottom">
    <div class="box" id="bottom">Bottom</div>
  </div>
</div>

Answer 2

I will do that this way...
simply with a double color (left,right) for background.

:root {
  --clear    : #e8eaff;
  --dark     : #18183a;
  }
* {
  margin     : 0;
  box-sizing : border-box;
  }
.container { 
  border-radius : 30px;
  width         : 200px;
  background    : linear-gradient(90deg, var(--clear) 50%, var(--dark) 50%);
  margin        : 2em;
  } 
.container > div {
  border-radius : inherit;
  padding       : 30px;
  }
.container > div:first-of-type {
  background : var(--dark);
  color      : var(--clear);
  height     : 150px;
  }
.container > div:last-of-type {
  background : var(--clear);
  color      : var(--dark);
  height     : 200px;
  }

<div class="container">
  <div>Top</div>
  <div>Bottom</div>
</div>