The data sources often exist to build these things yourself. For your example there is Open Food Facts and USDA FoodData Central.

https://world.openfoodfacts.org/

https://fdc.nal.usda.gov/

Both would make a great foundation for this sort of app. OFF is crowdsourced and does include barcode information. I have no idea how robust the dataset is for your geography though. If I were to build something like this for personal use, I'd be looking at a PWA that can leverage the camera for barcode scanning. I'd work with the existing crowd sourced database as well as provide a mechanism for "manual" entry which should just be scanning a barcode and taking a picture of the nutrition information. I've personally built systems like this before and all of these things are well within the capability of most SOTA LLM to build out.