“iirc helium is used in spacex starship raptor engines”
Helium is an inert gas. It is used in the space industry (where I work) as a purging agent. Different cryogenic fuels are used in the space industry, so when a system needs purging, you use a gas that will not freeze nor provide flammable fuel to the system. The only thing helium would be used for on a rocket engine is to purge the system of dangerous gases (Liquid Hydrogen, Liquid Oxygen, Liquid Methane, etc...)
We use helium in testing and calibration.
ok i am not in the industry. anyhow i believe helium is used in raptor spinup (this may or may not conflict with what you stated). i believe that sometimes an inert gas is needed for differential pressure reasons in valves. one side of the valve is kept filled with higher pressure liquid helium. the other side is a reactive liquid (eg methane or oxygen). this allows the valve seals to work to contain the reactive liquid(s). this is for full-flow staged-combustion-cycle engines. i believe russian full-flow staged-combustion-cycle engines might also use helium. basically spacex expanded on russian full-flow staged-combustion-cycle technology including valve seals. you may be able to comment. i read something about it a couple of years ago, but i cannot recall exactly where nor can i recall particulars.