The listing you are asking about is called a variation listing, it's for items in the same category and features a dropdown menu that buyers can select their items from.
You need to use eBay's new listing tool to set this up.
If you're already using eBay's new unified listing tool, at the very top of the page you’ll see the Photo & Video fields when you're setting up your listing.
Put in your title, then when you start to list your item you'll see the Photos & Video field at the very top.
The option to set variations is located between the Item Specifics and Condition sections, it'll appear once you've selected to list in Buy It Now format in a category that supports a multi variation listing.
If you still do not see the variation option between the Item Specifics and Condition sections, click on the 3 dots on the upper right hand side of the listing form and then select the settings gear icon and click on the variations toggle so it turns blue.
There are limited categories that support a variation style listing, if you're trying to list in a category or sub category that doesn't support this listing format, it won't appear, and your listings must be fixed price Buy It Now (not auction).
The following eBay page features information on multi variation listings:-
https://sellercentre.ebay.co.uk/business/multi-variation-listings
