If you need to loop over lists of data, use an each block:

<ul>
  {#each cats as cat}
    <li><a target="_blank" href="https://www.youtube.com/watch?v={cat.id}" rel="noreferrer">
      {cat.name}
    </a></li>
  {/each}
</ul>

The expression (cats, in this case) can be any array or array-like object (i.e. it has a length property). You can loop over generic iterables with each [...iterable].

You can get the current index as a second argument, like so:

{#each cats as cat, i}
  <li><a target="_blank" href="https://www.youtube.com/watch?v={cat.id}" rel="noreferrer">
    {i + 1}: {cat.name}
  </a></li>
{/each}

If you prefer, you can use destructuring — each cats as { id, name } — and replace cat.id and cat.name with id and name.