.... but, I've been told repeatedly that when someone KNOWS what they're doing with flash, you can't really tell they used it. I've yet to see an example of that myself, but I've had my eyes open for it. Until then, I prefer natural lighting.
So, window light, studio light, off camera Speedlite or on camera Speedlite?
I'll say... almost all of the above. I'm sure there was a window... but that probably didn't contribute to the lighting of the image primarily... I want to say some fill flash with on camera using a diffuser, and then off camera lighting... maybe a speedlite and umbrella to the left of the subject... so... maybe not studio light...