左右中央揃え

<div class="box box1"></div>

.box {
    width: 300px;
    height: 200px;
    background-color: #ccc;
  }

  .box1 {
    margin: 0 auto;
  }

<div class="box2-wrap">
  <div class="box box2"></div>
</div>

.box2-wrap {
  display: flex;
  gap: 10px;
  justify-content: center;
  align-items: center;
}

.box {
  width: 300px;
  height: 200px;
  margin-bottom: 20px !important;
  background-color: #ccc;
}

<div class="box3-wrap">
  <div class="box box3"></div>
</div>

.box3-wrap {
  width: 100%;
  height: 100%;
  position: relative;
}
.box {
  width: 300px;
  height: 200px;
  background-color: #ccc;
}
.box3 {
  top: 50%;
  left: 50%;
  position: absolute;
  transform: translate(-50%,-50%);
}

<div class="box4-wrap">
  <div class="box box4"></div>
</div>

.box3-wrap {
  width: 100%;
  height: 100%;

}
.box {
  width: 300px;
  height: 200px;
  background-color: #ccc;
}