PDF link of the entire thread:
Edina Digimap is available to all subscribing libraries and universities. It’s most useful if you want to download already geo-referenced OS maps.
For just viewing old OS maps, maps.nls.uk is the best place to go.
To learn more about how to use QGIS, I recommend following the Programming Historian tutorials – e.g. https://programminghistorian.org/en/lessons/qgis-layers