Different between Topspin and Sidespin serve

Most topspin serves will also have some sidespin on them. It all depends on the contact. To get pure topspin you need to make the ball spin only forwards.

ah I see.. so how to generate that topspin serve ? i must hit the back of the ball fast and then brush it up ? (to make a fast ball)

Yes the brushing up will give you the topspin.  Like anything doing it is the best way to improve it.

but, just like the "He Zhi Wen" serve  video, it seems like you don't brush the top of the ball rather than the side of the ball
so why can it be a topspin ? i confused as it's really fast just like topspin but i still don't know why it can be as you don't brush the top of the ball...

We are brushing up the back of the ball which also makes the ball rotate forward.

i've watched the video clearly and yes... you brush up the back of the ball
but, the contact of the ball is fine contact that you brush from the edge of the bat or flat that you hit the back of the ball flat and then brush it up ?

Flat and up.

oh i see

how to do that serve but make the ball short (but it still fast)

Keep it low and lots of topspin will get it down quickly after going over the net.