The screens included in iPhone replacement kits come from the original manufacturer, but some kits come with more tools to help remove the old screen and set the new one. However, many sellers use the same kits people use on eBay and other sites and sell at a significant margin.
Different iPhone models have different ways of removing the screen, but the design of most iPhones as of April 2015 requires users to melt the glue holding the screen in place and pry the screen away using a small plastic tool; repairs kits come with an appropriate tool, but a thin piece of plastic works as well.
After lifting the screen, it's often helpful to place a plastic tool underneath to keep it from adhering again, although people can also use guitar picks or other thin pieces of plastic. Repair kits often come with glue strips to make the screen easier to place, but people can also use glue designed for smartphone screens.
Repair kits rarely come with a heat gun or other tool for melting the glue that holds the screen in place. While it's possible to use a hair dryer, doing so takes far longer; people looking to replace multiple phones' screens might want to invest in a heat gun and learn how to use it without damaging other parts of the phone.