Cooked food / heat and serve meal, long term packaging

The commonest (in my experience - probably variable around the world) consumer DIY approach to this is the one that uses the step that you missed - high temperature (as mentioned by @John Feltz) - namely, pressure canning at 15 PSI (250°F) - in my experience, glass canning jars with metal lids, but flexible packages are evidently consumer-available more commonly that I've seen them around the world (perhaps even here, if I was looking for them.)

I've certainly eaten canned items stored at room temperature well older than 15 months and lived to tell the tale - indeed, I don't consider it anything to think about, other than verifying that the jar is, in fact, still sealed when time comes to use it. This may well not coincide with the most-paranoid recommendations on canned food shelf-life, but surely someone will be along to give the most-paranoid view shortly.

What they are using is canning. Canning does not depend on the container being glass; canning means that you

1. Have contents that are acidic enough to stop botulism spores from growing
2. Seal the food hermetically, so no new contamination can come in before the package is opened for consumption
3. Cook the food (in the container) at temperatures sufficient to kill everything inside except for botulism spores (they are very hardy, you can't get it hot enough to kill those)

If you have a plastic material which withstands the heat and pressure and seals hermetically, then you can can in it too. I am not aware of any consumer grade system which uses such material, but I have not searched for it specifically.

The only other way to preserve food for 15 months is freezing. There are no other options, unless you consider some which change the food itself (e.g. you can preserve meat by making pastrami out of it, or preserve cabbage by making sauerkraut).