Hi there,
Nice cut & paste job on the answer without giving an answer. (SPAM), map of portigaull. 
Let me try. Im not at my desk so this will be short.
For Nrf and Zephyr it seems complex but really isn’t once you get how the parts (files system) works.
The Device tree ,defines the hardware base of the SOC.
Your overlay sets the devices and type of devices you want to interact with or define as what you are using them for in your case its the PWM’s. The device tree defines how many you have available from the SOC (4)?
You can customize how it works and parameters with an Overlay file . I have a link to the best explanation of it. But
Lms if i can locate it.
I’ll edit the post.
There is a pwm blinky example, if you load that up you could see the DTS, and the dsti (i) is include.
Then the overlay. For tge nRF52840 DK.
It pwm speed controls the led faster and slower, with button press. I have a demo on here of it running.
HTH
GL 
PJ
Found it , go watch this it was very helpful.