<table>
  <thead>
    <tr>
      <th abbr="Maker">Manufacturer</th>
      <th abbr="Model">Car model</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Tesla</td>
      <td>3</td>
    </tr>
    <tr>
      <td>BYD</td>
      <td>Dolphin</td>
    </tr>
    <tr>
      <td>VW</td>
      <td>ID.3</td>
    </tr>
  </tbody>
</table>

const rows = document.querySelectorAll("thead tr");
const cells = rows[0].cells;

for (const cell of cells) {
  cell.textContent = `${cell.textContent} (${cell.abbr})`;
}