Usually once the truck is built it goes to a "Body Vendor" to have certain items installed. Its usually a team that all they do is install certain options. Yours might be painted ahead of time (same time as the cab of the truck) and then install the LED lights after the build. Once complete the window sticker will be available and the truck will be ready to ship once its released from the factory....pending any QA/QC inspections. I know for example my 2020 JGC Trailhawk had to go to a body vendor once it was built to have the Rock Rails installed. I'm not sure its a contractor or if its other FCA/Ram employees who install these.
EDIT:
@mxsnow might be able to answer this much better?