There are plenty of google and answers.ros hits on this but they all seem a bit complicated and also not necessarily up to the latest date. Here are some, of various vintages and from various sources. We're trying them but nothing is "smooth", hence this question

https://blog.roverrobotics.com/how-to-run-ros-on-startup-bootup/ https://roboticsbackend.com/make-ros-launch-start-on-boot-with-robot_upstart/ http://wiki.ros.org/robot_upstart

We are running ROS 1 Noetic on Ubuntu 20.04 on Turtlebot3s with Rasberry Pi 3+. We would like to create a ROS .launch file which is automatically run when the OS first starts up. This is not totally trivial because to ROS launch you need to have the environment set up correctly and also be the "correct" user; and there are probably other pre-requisites that I don't even know about.

What do people think is the "best" way to do this nowadays?