[QUESTION] Understanding how OpenCASCADE works

I am currently reading these books:




And i'm researching on how CAD software like OpenCASCADE works, i'm in the part of how extrusion algorithm works but i don't understand the algorithm given by the book. 

I would like to know how to sutdy the source code or how to search the functions implied in the operations like extrusion (but the source code is some complicated), i had read some about euler operators but i don't know if that is really used by OpenCASCADE.

Thank you and sorry for the english