Go to Theme Options - System - Install Child Theme.

Go to Appearance - Theme File Editor, and edit functions.php

Add this to the end of the page:

// Create related posts shortcode
function uicore_related_posts_shortcode( $atts = [] ) {
    if ( ! is_singular( 'post' ) ) {
        // Only show on single blog posts
        return '';
    }

    $atts = shortcode_atts( [
        'title'      => 'Related Posts',
        'count'      => 5,
        'orderby'    => 'date',   // 'date' | 'rand' | 'comment_count' etc.
        'order'      => 'DESC',
        'show_thumb' => 'false',  // 'true' or 'false'
        'class'      => '',       // extra wrapper class
    ], $atts, 'related_posts' );

    $post_id = get_the_ID();
    if ( ! $post_id ) return '';

    // Get current post category IDs
    $cat_ids = wp_get_post_terms( $post_id, 'category', [ 'fields' => 'ids' ] );
    if ( empty( $cat_ids ) ) return '';

    $q = new WP_Query( [
        'post_type'           => 'post',
        'post__not_in'        => [ $post_id ],
        'posts_per_page'      => (int) $atts['count'],
        'ignore_sticky_posts' => 1,
        'orderby'             => sanitize_key( $atts['orderby'] ),
        'order'               => $atts['order'] === 'ASC' ? 'ASC' : 'DESC',
        'tax_query'           => [
            [
                'taxonomy' => 'category',
                'field'    => 'term_id',
                'terms'    => $cat_ids,
            ],
        ],
    ] );

    if ( ! $q->have_posts() ) {
        wp_reset_postdata();
        return '';
    }

    $show_thumb = filter_var( $atts['show_thumb'], FILTER_VALIDATE_BOOLEAN );
    ob_start();
    ?>
    <aside class="uicore-related-posts <?php echo esc_attr( $atts['class'] ); ?>">
        <?php if ( ! empty( $atts['title'] ) ) : ?>
            <h3 class="uicore-related-posts__title"><?php echo esc_html( $atts['title'] ); ?></h3>
        <?php endif; ?>
        <ul class="uicore-related-posts__list">
            <?php while ( $q->have_posts() ) : $q->the_post(); ?>
                <li class="uicore-related-posts__item">
                    <a class="uicore-related-posts__link" href="<?php echo esc_url( get_permalink() ); ?>">
                        <?php if ( $show_thumb && has_post_thumbnail() ) : ?>
                            <span class="uicore-related-posts__thumb"><?php echo get_the_post_thumbnail( get_the_ID(), 'thumbnail', [ 'alt' => esc_attr( get_the_title() ) ] ); ?></span>
                        <?php endif; ?>
                        <span class="uicore-related-posts__text"><?php echo esc_html( get_the_title() ); ?></span>
                    </a>
                </li>
            <?php endwhile; ?>
        </ul>
    </aside>
    <?php
    wp_reset_postdata();
    return ob_get_clean();
}
add_shortcode( 'related_posts', 'uicore_related_posts_shortcode' );

Add this to Theme Options - Custom CSS:

.uicore-related-posts__list {
     list-style: none;
     margin: 0;
     padding: 0;
}
 .uicore-related-posts__item {
     display: flex;
     gap: .6rem;
     margin: .7rem 0;
}
 .uicore-related-posts__thumb img {
     display: block;
     width: 36px;
     height: 36px;
     object-fit: cover;
     border-radius: 8px;
}
 .uicore-related-posts__link {
     text-decoration: none;
     display: flex;
     align-items: center;
     gap: 10px;
}
 .uicore-related-posts__link:hover .uicore-related-posts__text {
     text-decoration: underline;
}

Go to Appearance - Widgets - Blog Post Sidebar, and add a shortcode block.

Usage examples