justify-content 主軸方向に子を寄せる

<div class="row">
  <span>A</span>
  <span>B</span>
</div>

.row {
  display: flex;
  justify-content: center;
  gap: 0.4rem;
  background: #f3f1ec;
  padding: 0.5rem;
}
.row > span {
  background: #efeef8;
  padding: 0.4rem 0.65rem;
}