Need help grasping some concepts

Hello, people.

I've been reading this list for quite a while now, and trying to understand what's going on, and wandering around in the code.

There are some concepts I'd like you to help me understand better:

1) What are plumbing and porcelain functions, methods, APIs... What's
the meaning of that classification and difference between the two?

2) What's going on about cache-tree not being able to handle empty
trees, or something like that? You mean Git doesn't store empty
directories in repositories?

3) And what are symbolic references? Are they just an alias I can
attach to branches and tags, like symbolic links in Linux that I can
attach to folders and files?

If you prefer, just a link to some material I can read to understand those concepts better would be great. :)

Thank you very much!