Hi Taka,
Japan is very much like the USA in the proprotions of electricity-generation sources, dominated by heat-power generation.
About 'dominated', which does it refer to; 'Japan' or 'the proprotions of electricity-generation sources'?
I've learned that it's a bit risky to offer the first opinion about your 'simple questions', but here goes.
I don't see the sentence as well written.
I think 'dominated' is meant to refer to 'sources', since surely only sources can be dominated by one kind of source? However, if it refers to sources, I suggest the comma should be omitted.
I wouldn't even try to read it as referring just to Japan. If you wanted to go in that direction, I think you'd have to try to read it as referring to both Japan and the USA, since they are both linked in the thought.
The main thing that bothers me, that I find confuses the meaning, is the use of the word 'proportions'. If you speak of this, you need to speak of two things, of 'the proportion of A to B'. What are the two things here? Does it mean that Japan and the USA have the same number of such sources? Then just say number. Or does it mean that, in Japan, the proportion of electricity sources to grammarians is the same as the like proportion in the USA? The sentence is very lacking in information to explain what it means by 'proportion'.
Let's see what other thoughts people have to offer.
Best wishes, Clive