This is a business inquiry as spoken about here
I am designing a custom Uno which I hope to sell at some point. I have included the ATMEGA16u2 as the USB chip, but I don't know if I need the ICSP header that is included on the original Uno for programming the 16u2. Does the ATMEGA16u2 need to be programmed before it is used in this manner, or does it work out of the box?
Yes.
Your board supplier should also be able to do it for you, either in circuit or before soldering the chip.
Does that mean I need the ICSP header, or can I program it before hand? Is it a bad idea to use a CH340 chip instead? Does it need programming?
Do they need the ICSP header?
EDIT: No, they don't. And yes, the company I use does program the chips for me. But - what do I tell them to program it with?? What code??
I think I'm in over my head 
Can I just send them this??
ArduinoCore-avr/firmwares/atmegaxxu2 at master · arduino/ArduinoCore-avr · GitHub
Also, can I sell a board with that code on it? The website says "No devices
using this VID/PID combination may be released to the general public."
I'm not sure; there is something called Atmel flip but not sure how it works; it's my understanding that you only have to ground a pin of the 16U2 to get it in a mode in which you can upload the firmware. https://ww1.microchip.com/downloads/en/Appnotes/doc8457.pdf
It's not a bad idea and it does not need programming. But I'm not sure if you can get your hands on them. FTDI chips might be a better option.
I don't know which one I would try; on a Windows PC you can find them here:
I don't know anything about legal stuff. I think that you will have to ask permission / take a license and probably pay for the use. A VID is a Vendor ID identifying the vendor and you're not that specific vendor.
Maybe I'll use these:
Cheap as chips 
I'm not sure which FTDI chip I would use. Do you have any ideas?
The original Nano uses a a FTDI chip; Sparkfun's RedBoard also uses a FTDI chip; look at their schematics.
Probably if you take a million of them 
I did not check
I think yes, you need it.
During experiments with the board, the user can accidentally ruin the firmware and then the board without a ICSP header will turn into a brick
To buy one is $0.61 excluding shipping. About 13 times cheaper than the ATMEGA16u2!
Will do, thanks
Are you hoping to make a board cheaper than the chinese analogs? 
The chinese are producing nano boards with CH340 by the millions
No hope of that!! No, I'm making a Uno sized board that has a couple changes.
However, I do need to be able to afford a prototype to test!!
It uses the CH340C chip.
No, it uses a FTDI chip; see http://cdn.sparkfun.com/datasheets/Dev/Arduino/Boards/RedBoard-V22.pdf
Ah, I see. I'm going to use FT232RL I think.
That board is dirty
Will this fuse work for the polyfuse?
