at what step regex is applied

hi everyone
i am new in busines and cant get throught the conversion :smack:
i ve read the EBook conversion tutorial
i got that the are 4 steps
but i dont get it where do the Structure detection XPath and Regex is applied

i mean i ve got in the debug catalog 4 catalogs
-input
- parsed
- processed
- structure
all of them have index.html with all the text of the ebook
but as i guess only one of them is used
which one ?
and one more question
is the regex replace executed before XPath , or opposite ?