One of the founders of Ferrous Systems has answered some questions about it on Hacker News. See here and here.

The spec they created for the certification process is open source. There is some “tiny” amount of the patches that aren’t public but it sounds like it is essentially a recent stable release of Rust because the other major changes have been contributed upstream. It’s not clear if they definitely plan to eventually release the rest of thier changes as open source or not but they will consider it.

I recommend Waistline if you use Android. It’s on F-Droid.

The barcode scanner uses a community run database (Open Food Facts) for nutrition facts so you might find that you need to manually input items or correct outdated info more often than compared to the more popular My Fitness Pal. Obviously that gets easier once you build up a collection of your usual foods in the app.