I don’t know what C……9 is.
That’s a very broad question. Yes a properly made faraday cage keeps out radio frequency. Electronic engineers know more about them. My understanding is that the stronger the radio frequency, the thicker the metal has to be.
You can do a double layer insulated faraday cage for highly important things:
First layer is maybe wrap the device 3 times with aluminum foil.
Then put a non-conductive material around that so that it doesn’t touch the outer faraday cage.
Then put another sealed metal container around it object and non-conductive material. Could be a metal filing cabinet (possibly with the gaps covered with metal tape? Or a metal trashcan with well fitting metal lid, etc.
There are DIY versions, but if you want something more portable and pretty I guess people want to buy it pre-made. To test them to see if they actually work you put your cellphone turned on in the faraday cage and see if you can call it.