True Seeing spell allows, among other things, this:
and can see into the Ethereal Plane, all out to a range of 120 feet.
Assume that True Seeing doesn't allow one to directly see through the Fog Cloud fog, which is actual conjured fog and not an illusion, and creates a heavily obscured area. I believe this is a common ruling, not a contested topic, and anyway this whole question becomes moot otherwise.
But, Fog Cloud doesn't say it extends to the Ethereal Plane. So, considering above, what does the creature under True Seeing effect see in a fog cloud? Most importantly, does this give any advantage, be it for fighting or moving or anything, over other creatures in the Fog Cloud?
Or, is it simply so, that what ever may be seen on the Ethereal Plane does not reflect what is going on in the Material Plane? Or the fog in fact does heavily obscure the Ethereal Plane, too? Or what?
I suppose related matter is, how are permanent obstacles like walls seen with True Seeing on the Ethereal Plane. My understanding is that they exist as "ethereal material" there, but I am not sure. Fog Cloud creates a temporary spell effect on the Material Plane though, so possibly different.
If this question can't be answered within 5e material, drawing from earlier edition rules or lore is ok.