But exactly what is a "half step" "single step" "double step" let alone an "alpine"?
Could you explain please - or link me to somewhere it's explained ?
Half step was used back when the rear was only 5 or 6 speed.
If you had a 14-28 5-speed block, the interval between gears is about 20%.
The idea was that you would have 2 chainrings 10% apart, to split the difference in a rear shift.
You'd end up shifting at the front every shift, but you'd get smaller gaps between gears.
Going down from top, you'd have a sequence like ...
48/14, 44/14, 48/17, 44/17, 48/20...
You could get special front mechs to handle the small difference between chainrings.
"Half step plus granny" was also a thing - a triple like 48/44/24, which gave close ratio cruising gears without losing the climbing gears.
"Alpine" is a smaller than compact double - something like 42/28.
It gives the climbing gears for going up an Alp, and the expectation is that you aren't racing so 42/11 is high enough for the valley roads.