This depends on the particular manufacturer and the particular model you are looking at. Many external GPUs do come with built-in GPUs such as the Gigabyte AORUS Gaming Box, while others such as the Razer Core X are just empty enclosures that would require you to install your own graphics card. The value proposition also varies widely between these two options, as it is sometimes cheaper to get a box with a graphics card built-in as opposed to having to purchase a card separately.