I won't quibble over the difference between the
Su-35 you referred to, and the Su-30 pictures you linked, given the confusion in numbering, and that they're both developments of the Su-27. I will just say, damn that Sukhoi design group sure does have a distinctive style!
The PRC is being aggressive because they think they can get away with it. The USA is massively over-committed, and being far too aggressive with too many different people in too many different places. This in itself is rather perplexing behavior. Either Uncle Sam has finally gone totally senile, or he has one hell of an ace up his sleeve. It's enough to make one wonder about orbital weapons, and whether those "meteors" that hit Russia recently were really meteors, or a demonstration of capability. If the latter, the bear doesn't seem to be much intimidated.
So the PRC is flexing its muscle now, while they have a window of opportunity, or think they do. After all, possession is 9/10's of the law, and if China can present the West with a
fait accompli in the South and East China Sea, what is the West going to do about it? Asian politics being what they are, the little tigers will all fall in line if China appears to be winning. And it's not like Malaysia, Vietnam or Brunei can do anything.
Japan has made a serious mistake in depending upon the USA for its defense.