How do I highlight or snippet an attribute that is an array of strings?

With InstantSearch, you can leverage the item template. This template receives an object containing a single record. On the hit object you can access the _highlightResult and _snippetResult

instantsearch.widgets.hits({
  container: '#hits',
  templates: {
    item(hit) {
      return `<article>
      <b>Highlight Arrays</b>
      <ul>${hit._highlightResult.categories
        .map(({ value }) => `<li>${value}</li>`)
        .join('')}</ul>
      <b>Snippet Arrays</b>
      <ul>${hit._snippetResult.categories
        .map(({ value }) => `<li>${value}</li>`)
        .join('')}</ul>
     </article>`;
    },
  },
}),

• Working Demo
• Documentation Reference