html

    <!-- 폰트오썸 아이콘 링크 -->
    <link
      rel="stylesheet"
      href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.4.2/css/all.min.css"
    />

    <div class="search">
      <input type="text" class="input" placeholder="검색..." />
      <button class="btn">
        <i class="fas fa-search"></i>
      </button>
    </div>

css

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

* {
  box-sizing: border-box;
}

body {
  background-image: linear-gradient(90deg, #7d5fff, #7158e2);
  font-family: 'Single Day', cursive;
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
  overflow: hidden;
  margin: 0;
}

.search {
  position: relative;
  height: 50px;
}

.search .input {
  background-color: #fff;
  border: 0;
  font-size: 18px;
  padding: 15px;
  height: 50px;
  width: 50px;
  transition: width 0.3s ease;
}

.btn {
  background-color: #fff;
  border: 0;
  cursor: pointer;
  font-size: 24px;
  position: absolute;
  top: 0;
  left: 0;
  height: 50px;
  width: 50px;
  transition: transform 0.3s ease;
}

.btn:focus,
.input:focus {
  outline: none;
}

.search.active .input {
  width: 200px;
}

.search.active .btn {
  transform: translateX(198px);
}