Thanks for the nice pictures from some of my preferred roads that I try to ride at least once per year.
The bunkers you saw on top of the Bonnette road were part of the Maginot line from WWII. It is not well known but the Maginot line was very efficient in the Alps against the Italian army which attacked AFTER the French army was nearly completely destroyed by the German army.
These specific bunkers are https://en.wikipedia.org/wiki/Ouvrage_Restefond
There are a lot of the Maginot forts in the Alps and there are always in beautiful places. Some are restored by associations and open for visit.