html

<label id="label1" for="test">Label 1</label>
<button id="test">Button</button>
<label id="label2" for="test">Label 2</label>

js

window.addEventListener("DOMContentLoaded", () => {
  const button = document.getElementById("test");
  for (const label of button.labels) {
    console.log(label.textContent); // "Label 1" and "Label 2"
  }
});