http://dds6qkxpeiqvogt4y2fd7vtsxq6pvokekb2h4la7jd4iwl4ljuawu5qd.onion/wiki/Dev/Technical_Introduction
If you are more paranoid or have higher security needs, read everything, full documentation and full technical design, you'll learn about physical isolation and build Whonix from source code and so on. And no, Whonix does not claim to protect from very powerful adversaries, to be a perfectly secure system, to provide strong anonymity, or to provide protection from technically advanced surveillance and similar.