Hello,

In the “Product data” section, make sure that “Variable product” is selected as the product type.
After selecting the “Variable product” type, you will see a new tab called “Variations”. Click on it.
On the “Variations” tab, you will find a list of all the variations for the product.

Locate the variation that is out of stock and click on “Edit” next to it.

In the variation settings, you should see an option called “Manage stock”. Check this option.
Once you check the “Manage stock” option, additional fields will appear, including “Stock status” and “Stock quantity”.

Please try this custom CSS code and paste it to Theme Settings >> Custom CSS >> Global CSS section:

button.single_add_to_cart_button.button.alt {
    flex: 1 0 auto;
}

Go to Theme Setting -> Single Product -> Related & Upsells. By default Related products are products of the same product category,

Please check this article: https://docs.woocommerce.com/document/related-products-up-sells-and-cross-sells/

Best Regards.