for xwindows (buster), new location:

sudo vi /etc/xdg/lxsession/LXDE-pi/autostart

recommanded to run bash script from autostart. add at the bottom:

bash start.sh

example bash:

!/bin/bash

sleep 5
chromium-browser --start-fullscreen /media/pi/P/index.html &
sleep 10
xdotool key ctrl+R