The TurtleBot4 software and and hardware are documented in the turtlebot organization. You can try it out in simulation already!
If you search for turtlebot4 you’ll find 10 different repositories. Including hardware, simulation, lessons and user manual source.
The best entry point for learning about the technical side is the user manual.
We’ve mostly been testing from source, however the first packages are in the testing repository of Galactic: ROS packages for Galactic - 2022-05-09 23:00:28 -0800 The are actively being released to be ready for when the preorders are delivered. And we are shooting to push it forward into Humble asap to support that as the primary target platform on the LTS.