Can someone give me a tip on where to set a Convention Center, Stock Exchange and other reward buildings so that they can be in good use. and does it mean that having the convention center and the stock exchange, courthouse, bereau of bereau of bereaucracy can skyscrapers start to appear??

they can help get skyscrapers...

i usually put them in a more-heavy commercial area.

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set them in areas where you want more denser commerical to develop around them.

They can help a lot, but really you're better off relying on individual plazas, because they really add up in benefitting you and don't have "side-effects" like crime or garbage.