Rocket Propellent Injectors are critical parts of the engine design, they take the propellents and mix them so that they can quickly burn in the combustion chamber. Injectors can make or (literally) break a rocket design, and over the years we've seen rocket engines move from injector plate designs to more efficient options as engineers have come to understand what works well.

Thanks to Copenhagen Suborbitals for sharing some video of their injectors being tested, I hope get get to see some more flights with these:
https://copenhagensuborbitals.com/