I have downloaded the Archi source code and I would like to understand how it was developed (starting from mode to code generation and so on). Is there a document that explains this logic?
not sure what you mean by "mode".
As for code generation, only the model classes (the "com.archimatetool.model" package) are generated as basic framework classes, which have a lot of additional logic code added. All the other code is not generated and is based on the Eclipse Rich Client Platform, GEF, Draw2D and EMF. You'd need to read the available documentation for these frameworks to understand how it all fits together.