It is unclear how exactly this feature will work, but presumably it could be similar to other apps such as MyFitnessPal. For those unfamiliar, MyFitnessPal is a health tracking app that lets users log their daily meals and also helps them count calories. It can also be used to scan barcodes for products you buy off the shelf so you get more accurate nutrition information.
https://twitter.com/connorjewiss/status/1397158493405302785
That would be one way that Apple could introduce food tracking to iOS 15 through the Health app, but as some have pointed out in the comments, MyFitnessPal has managed to build up quite a massive database of food items over the years, so it will be interesting to see how Apple could compete with them.
In any case, it would be interesting to see such a feature arrive for iOS devices. We expect more details about iOS 15 will be revealed in the coming weeks as WWDC 2021 is around the corner, so check back with us then for more updates.