where are my iphone backup files?

The thing is that there are some (actually many) files that one should not mess with. Examples are generally those files in the Library directory and its subdirectories. Apple may attach other structures to these files, and/or relocate these files or change their format in subsequent software releases. If you try to go around Apple's internal handling of these files, you are only inviting future problems.

If you have to know where the backup files are, simply open iTunes, choose iTunes > Preferences > Devices from iTunes' menu, control-click on the iPhone backup listed there, and then click on "Reveal in Finder". But again, I caution you about doing this, moving or opening the files, etc.

I wonder why this matters to you. It's best to access these files using iTunes itself.