Is there any list of all commands and what they do/mean?
for things like $replace, $read, $3, $chr ???

Hi,

open up mIRC and type /help, then click on the second button on top called "Index".
You can also use /help like this: /help <keyword> and the help file will popup in the right category.

Note that you'll have to scroll sometimes through the help file, because unfortunately, mIRC's helpfile doen't take you straight to the right topic.
Example: /help $read

Greets