changing http headers after output

changing http headers after output

I have a few questions regarding headers and output buffering.
I know headers must be send before output or they will not work, and that
output buffering stores all HTML into a buffer and sends it as one as
opposed to PHP processes sent bits at a time.
So does this mean when output buffering is on, all content is collected
into one variable and where ever the headers were defined in the script
they will be placed at the top/first?
And if output buffering is off you have to declare headers before any output?
And also to use any output buffering functions such as ob_clean() you need
output buffering to be on? as if output buffering was off, you couldn't
clean, 'take back', anything as it would of already been sent?
Finally is output buffering turned on/off within php.ini? as my XAMPP
local host server seems to have output buffering on and my VPS doesn't,
meaning I need to go to my VPS php.ini?