How to search for categories and/or tags?

Here's a function that will search for any categories (or other taxonomies) that match the given string and then returns all the posts that are included in that category.

function searchTermPosts(String $query) {
    // First get the categories / taxonomies that have a 'name like' the query
    $terms = get_terms([ "name__like" => $query ]);

    // Now convert to a taxonomy query we can use in WP_Query
    $tax_query = array_map(function ($term) {
        return [
            "taxonomy" => $term->taxonomy,
            "terms" => $term->term_taxonomy_id
        ];
    }, $terms);

    // Add an "OR" clause to find posts in all categories
    $tax_query["relation"] = "OR";

    // Now do the query
    $results = new WP_Query([
        "tax_query" => $tax_query
    ]);

    // Return both results and the terms, as well as term names
    return [
        "results" => $results,
        "terms" => $terms,
        "term_names" => array_map(fn ($t) => $t->name, $terms)
    ];
}

And then use it like this:

$query = searchTermPosts("fruits");
$count = $query['results']->found_posts;
$cats = implode(",", $query['term_names']);

echo "Found $count results in these categories: $cats";
echo "<ul>";

while ($query['results']->have_posts()) {
    $query['results']->the_post();
    echo '<li><a href="' . get_the_permalink() . '">';
    the_title();
    echo '</a></li>';
}

echo "</ul>";

// Above the loop on your search result template:
if ( is_search() ) // Are we on a search result page?
{
    global $wp_query, $query_string;
    // now modify/filter the query (result)
    query_posts( $query_string . 'cat=1&tag=apples+apples' );
}