misterroy wrote

Am I correct in thinking that large wind turbines do not
produce power when the wind speed is below 12mph?

Nope, you are wrong about that.

Why on some days do they stop altogether and others they
keep turning, even though there is hardly any wind at all?

The wind can be surprisingly variable, particularly
when there is hardly any wind at all.

Just been out on a mates 2 ton yacht, testing the launching
system and its surprising how variable the wind can be.