Hello,

Sorry for the delay.

1. Such field you can create by using the variable product (Dashboard -> Products).

https://monosnap.com/file/FuXZ7ogQKa6kAU5nGe5jG09XPJqMz9

First of all, create attributes (Dashboard -> Products -> Attributes) and add them to the appropriate product.

https://gyazo.com/715d715e52decb3ecbbf48628da48515

https://monosnap.com/file/ZxVKJo7ahnrk62BatCHPXNkzChCDXQ

In the end, you will see such a dropdown as in the below video.

https://gyazo.com/3ebec48f92e2c1f8d0ad3af978314614

Here is our documentation on how to create variable products and swatches.

https://xtemos.com/docs-topic/variable-products-and-swatches-2/

2. As for the site example, they are using the “Extra product options For WooCommerce” plugin for showing the dropdown.

https://wordpress.org/plugins/woo-extra-product-options/

Kind Regards