Having trouble connecting to your OAK-D camera on the TurtleBot4? It might need UDEV rules for the device in order for ROS to be able to see it.
A quick tutorial on how to create a ROS Overlay to modify the launch file for TurtleBot4 Lite in order to enable joystick control with a Bluetooth controller.
Are you trying to figure out how ROS starts up when the TurtleBot4 boots up? This article explains how the ROS Launch files are used to start up ROS on the robot using the robot_upstart package to create a Linux service.