Need some help here. I saw this feature on a website and now I'm trying to find what app that can do the same for me or if this is a build-in function in a theme. It's a Shopify website.
I believe Bundle Kit does exactly what you need!
Our layout is not exactly like your example, however, customer will see each bundle as a Standalone Product (see an example below).
Bundle Kit tracks the inventory levels of the bundle and the individual bundled items as they are sold in real-time to prevent overselling.
You can also build a bundle with multiple selectable variants each containing different products of different quantities.
You can try the app for free at: https://apps.shopify.com/bundle-kit
Hope this helps!