third party plugin loader, need content catching

First of, thank you for reading the question to begin with.
For sum people its already quite an achievement.

Secondly, back to point.

I am building a third party plugin, indepedent from the wordpress plugin system.However, the variable I need, seems to be exported already, which is the complete content.I need to catch it, and prevent it from exporting, and export it later on.My idea was to put the content normally exported in html to browser, into variable, which is kinda what I need.
Now It could be just me that I have been looking over these codes, but whatever the case, I couldnt find it.

My questions here are:
in what file(s) is the html content generated/retrieved?
in what file(s) is the html content printed or echo’d, or whatever function is used to export the code to the browser?

Sidenotes:
1:I am experienced php,html c++ programmer, just sometimes littlebit confused about someone else his or her work.Therefore the questions.
2:I am using a clean installation of wordpress, unmotified straight from wordpress.org