To customize this, create your own in your home directory. Name it .xinitrc, make sure it is an executable script, and chmod +x.
– TLDP - The X Window User HOWTO - Running X
Customization is done by a .xinitrc file in ~; if present, /etc/X11/xinit/xinitrc doesn't load.
The above TLDP HOWTO suggests that there is .xclients for situations without .xinitrc.
The order of loading the files works like this:
Startx typically runs without command line arguments, but command line arguments will override its normal behavior described below. It gets client arguments from one of:
.xinitrc in the user's home directory, if the file exists.
/etc/X11/xinit/xinitrc if the above file doesn't exist.
/usr/X11R6/lib/X11/xinit/xinitrc by default if the above two files don't exist.
– How Linux X works
As the quote reveals; startx decides this, which is a shell script that can be customized.