where is the iPhone backup file located on the PC?

Locate backups of your iPhone, iPad, and iPod touch - Apple Support

Of course, you should be extremely careful about accessing this file directly. That is not the intended access method.

An iOS backup is not a file. It is a SQLite database; it is a folder with tens of thousands of files, one file for each data item. My most recent backup is 43,000 files, for example.