If I'd had to take a guess they're outsourced. I've worked at consultancies that provided for manufacturing, retail, etc. They may or may not have a thin internal presence and then the rest are contractors from the outsourcing company like Accenture. B2B / wholesale manufacturers wouldn't need a digital presence or anything like that.
I was at a Supermarket once, looked at the cash register (that had a screen) and the back of it had a Netsuite logo - so clearly outsourced.
This is classics of embedded jobs.
The machine that packs your milk isn't produced by the dairy, and the machine that makes circuit boards isn't made by the company that makes circuit boards. They are produced by companies that makes those machines. These machines are a subset of "embedded systems". I regularly see job postings for such jobs (not on HN, but places like Indeed). This is often C++ programming, but there may be some Rust or Ada or straight C (even some assembly).
Supporting the supply chain is different. I'm sure there's a term for this area. (ERP, maybe? Does that fit?) I think a lot of this is Java coding, with (often) SQL on the back end. Again, though, the dairy or whatever doesn't usually write that software. They buy it, either from an ERP vendor or a consultancy. And again, yes, those jobs do appear, though perhaps using different words than you are expecting.