I have been using QGIS and its various updates in a limited way to make maps for a few years now, but I have always had the same problem with the Print Composer. I make my layered maps with grouped annotations and save them as templates, which I'd always assumed would allow me to recall them at another time, when the map would pop up just as I'd left it and if I wanted to make minor changes or adjustments, I could then do so. This has never happened. What does happen is that when I open Print Composer files, the annotations appear, but not the original map images. I have looked at various QGIS manuals but cannot find the answer to this. Would someone be able to explain to me if there is a way to save the complete content of maps for retrieval and/or editing at a future date? To me, this would appear to be the main function of a GIS, so I must be misunderstanding how it works.
It's seems like you have indeed misunderstood the use of Composer templates. A template does not save any spatial data. A template only stores the Print Composer layout: the map, legend, label and other items which make up the layout and their settings.
If you want to reuse project layers (spatial data and associated styling), then you should look into the embed layers functionality. This functionality allows you to have projects with prepared layers which you can then import (or "embed") into other projects. These embedded layers will be read-only, which means that you can turn them on and off but you cannot change the layer style.
Another approach you can try, is to save a default style for your layers. Whenever you load the same layer into a different project, QGIS will render it in this default style. Afterwards you can still adjust the style if you want to.