I'm looking for information about the default Magento theme including:
Explanation of all directories.
Explanation of files per directory.
List & Definition of PHP snippets per
file. For example, the PHP snippets
used to place the buy button, review
starts, product images, etc
Does anybody know if/where I can find this?
My suggestion is to read - and do not expect easy answers on here. You are dealing with one of the most complicated Open Source Shopping Cart systems out there. If you are having difficulties figuring out how to place the Buy Button, you are going to be crying by next week.
There is a lot involved when setting up a E-Commerce site. In my case, a lot of swearing as well. Good luck.
What you want doesn't exist. There is no "skinning guide" for Magento. The closest thing you'll find is the Designer's Guide. If you're interested in learning how the layout engine works from a programming point of view I wrote an article that covers that, but you're not going to find a robust "here's how to skin Magento in thirty seven easy steps".
There's a steep learning curve with Magento, and no way NOT to spend a "ridiculous amount of time" on your first project. Given how hard it is to find competent Magento developers, consider it an investment in the platform.