To customize the product page, go to Customizer → WooCommerce → Product Page.
The related products are picked automatically based on the product category and/or tags they belong to.
You can also recommend products manually by linking cross-sell products. If a product has up-sell products linked to it, they will show on that product page regardless if the Related Products setting is toggled on or off (this toggle only affects the automatically recommended products).
Showing automatically related products (Related Products toggle enabled) and manually linked cross-sells will look like this:
You link up-sell products by editing the product:
Read more about related products, up-sells and cross-sells in this woocommerce.com article.