I have been a PHP developer for 13+ years, have extensive knowledge of PHP itself as long as some other popular projects like WordPress. I have no experience with Magento.

I now have a full time job working for a company that uses Magento, all my development work will be on Magento pretty much now.

My initial thoughts after looking at Magento for the first time are, wow what have I got myself into now? But then the experienced developer in me comes out and remembers, it's just PHP.

So I have some basic Magento questions I would really appreciate some answers to from more experienced Magento Developers...

1. What is the best most useful places to learn about magento development beside this site?
2. Extensions/plugins are very confusing so far to me. Other platforms like WordPress allow you to place a plugin into it's own folder and all it's files will live under that folder. From what I have seen so far, a plugin might be scattered accross several folders in Magento, is this correct?
3. I have learned about how to over-ride Core functionality. So if a file in the core is located here /app/code/core/Mage/SitemapModel/Resource/Catalog then I can over-ride it by creating the file in this location /app/code/local/Mage/SitemapModel/Resource/Catalog so how does this work with an Extension/plugin? If I create a plugin that need to over-ride core functionality, then I must have my files all over the place for that extension to work?
4. Any other info you wish you knew starting out with magento?

Thanks for any insight, I know this is a multi part question but I feel any of the answers would be helpful to myself and other starting out, I will make this public wiki if I must as well.