Graham bemethavinci _headtags is just a plain text file that holds the css links and stuff like the xt:auth function
_header is your navigation so look in the tutorial under navbar and nav
bootstrap is a responsive template not a fixed width one but I don't think you meant that
This forum uses a 'fixed' template (it looks the same on both mobile and web)
the container grid and column bootstrap components take care of the screen sizes
again look at the tutorials