Nick here from Shopify. Good question!
This is difficult to tell without seeing the website itself, even seeing the website, I wouldn't be able to confirm if it is a Shopify website or not for privacy and security reasons. If it's not an app which you aren't sure it is, it looks like it could be something customised by a third party developer, but without seeing it, it is really difficult to say for certain how they have it this way. Something you could do is reach out to the website directly and ask them? A lot of store owners are open to chatting about aspects of their store like this and that would give you your answer for sure.
It could also be a product options app from the Shopify App store and not a bundle app. Product option apps have the ability to offer a wide range of options for a product which looks similar to the colour and size options shown in the screenshot you shared. Do you think that could be more what you're looking for?
All the best, Nick