Welcome to geekhack :)
I don't know which kit you're looking at but a kit always includes a case, plate and PCB; it may also include switches, stabilisers and sometimes even keycaps - you need all of these (except the plate) to build a working keyboard. If you need a separate USB C module I would hope it was available on the same site and that it had mounting holes and Swills builder has a 'custom polygon' section for you to add matching ones.
I found a usb-c module with JST cables, now where do I attach it?
Here a close up of the pcb that includes the part of interest (from what I understand).
More
(https://ae01.alicdn.com/kf/U31f23ea4f7b94fcf98935fd16166b338O.jpg)
Also the battery, where does it go?