A question about cutscenes.

[DevelopSheep]

In which way cutscenes are played in GTA SA? 
I tried to open the main.scm through the SannyBuilder, and sadly haven't found any line linked with cutscenes
Where is the cutscenes played? How they are played, when we start a certain mission, how can i change it for example a cutscene from GTA 3 or GTA LCS 
and play it in GTA SA before the certain mission?
 

I've searched all the google, but sadly haven't found the answer on my question, will be very thankful if someone here will tell me how to do those things :)))