Hey Oleg,
a) How does the wyBuild know what directory to put the new files in?
The "Your program's folder" in wyBuild corresponds to the folder where "wyUpdate.exe" and the "client.wyc" files are sitting. For instance, if you install your software to "C:\Program Files\YourApp", you'll put the wyUpdate.exe and client.wyc files in this "YourApp" folder.
b) If my only update is one file, can I just specify that file only, or do I have to specify ALL files that are currently in my install (about 75 files total)?
If you follow the Step-by-step walkthrough article you see we tell you to add all your files to wyBuild. Then, when you create the updates, wyBuild compares the versions of your software and generates the smallest possible patch.
Tell me if this helps.