New PCB Firmware

Hardware Newbie here....
If I take the Arduino, say Uno, opensource Eagle/Gerber files and have a PCB manufacturer build the boards, what other task are needed to get the boards up and running? I assume that there will need to be firmware loaded. If so, where do I find that firmware and how do I load it? I just have no concept of how much effort is necessary to take a new board from the maunfacturer that build the boards from these PCB Eagle files and get them up and functional.

You will need to set the fuses on the microcontroller and flash the bootloader. You can learn about that here:

For volume manufacturing, you can provide the supplier of the microcontrollers with the firmware and have them program it, but I'd guess there is some minimum, below which you'll end up just burning the bootloaders yourself.

pert:
You will need to set the fuses on the microcontroller and flash the bootloader. You can learn about that here:
https://www.arduino.cc/en/Tutorial/BuiltInExamples/ArduinoISP

For volume manufacturing, you can provide the supplier of the microcontrollers with the firmware and have them program it, but I'd guess there is some minimum, below which you'll end up just burning the bootloaders yourself.

Actually, Digikey is well prepared to do any programming you like. Even one-off. For example, we had a customer with a very old product they needed to resurrect. The only code the customer had was already on an old eprom. We sent the eprom and a new eprom and they copied the code, successfully, to the new chip and the customer was very happy and built many more of the old stuff and Digikey got to program chips for the remainder of the life of the product.
Paul