<div class="wrap">
  <div class="text">Lorem, ipsum.</div>
  <button>btn</button>
</div>

.wrap {
  display: grid;
  grid-template-columns: auto 1fr auto;
  gap: 3px;
}

.wrap::after {
  align-self: end;
  content: "";
  height: 2px;
  background: red;
}

button {
  order: 2;
}