Different home pages based on different conditions? I have two pages:

/home
/pr


Currently, under site configuration, I have /home as the home page. So whenever someone open mysite1.com, it will show the contents at /home. However, multiple domains are parked on this site. I want it in the case of mysite2.com is opened, the /pr page must show, but if mysite1.com is opened, it must show /home like it always does.

At what point would I be able to do this? Where  can I incorporate this logic?
You could do this in settings.php and define $conf['site_frontpage'] from a server variable like $_SERVER['HTTP_HOST']. The same logic would allow you to set your $base_url(D7) or $settings['trusted_host_patterns'](D8) to the correct domain as well.