The page footer is output here: https://github.com/moodle/moodle/blob/master/lib/outputrenderers.php#L1167
But you don't want to be messing around with it there - if you need to override the functionality, you probably want to be writing a renderer override in your theme and using that to change the functionality of the footer output.
See https://docs.moodle.org/dev/Overriding_a_renderer for more details.