Skill Engine Structure

[Horus]

Good evening,

Today I was trying to see the overall structure of the skills engine, however it turned out to be a more complicated task than I had anticipated.
Would someone be kind enough to describe how exactly does it work? Meaning, what class does what.

I do realize this is a rather tedious and annoying question to ask.

Thank you in advance.

Sincerely,

Horus.