Understanding Definition Information for Classes Referenced by Swift

When using xcode and writing code with swift
You can see class definition information by clicking command + class name.
Where is that definition file?
(You can find the header file of Objective-C, but you can't find the important swift...)

The intention is to simply review the applications of classes and functions.
Rather than looking at the document, I thought it would be easier if I could see the definition file directly.