How do I install can-less down lights in new construction?
It's pretty straightforward.  These lights require a junction box, but they provide it.  The junction box includes the driver for the LEDs.  The spur cable going from the "j-box" to the LED itself is low voltage DC.  

You can see the j-box on the right here, and there's a closeup showing the color temperature switch.  



The "j-box" and LED head are mates; you can swap LEDs among J-boxes, but cannot just throw the "j-box" in the trash and connect 120VAC straight into the LED cable. Won't work. 

You are meant to be able to access this junction box by unmounting it (if your jurisdiction requires mounting it) and pulling the "j-box" down through the LED hole so you can get your hands on the wires.  Weird, but shrug UL approved it that way.