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.
Sounds like you absorbed it pretty well. This handy temperature reference will help explain it’s uses:
Liquid Helium (LHe) has a temperature around -452F. Inert
Liquid Hydrogen (LH2) has a temperature around -423F. Extremely flammable.
Liquid Nitrogen (LN2) has a temperature around -320F. Inert
Liquid Oxygen (LOX) has a temperature around -297F. EXTREMELY Flammable. It is used as the oxidizer for other cryogenic fuels.
Liquid Methanes (there are different types) have a temperature near that of LOX.
System Purges are done with Gaseous Helium to help remove the the flammable fuels from the system. Helium is good to do that with because it’s liquid point is so low.
Also, some control valving used on the flammable cryogenic storage tanks use pressurized Helium because it introduces no electrical spark issues and will not have problem turning into liquid because it has a lower temperature.
Not a subject-matter expert in the field but have had to use LHe turn run some tests on super-cooled devices, so we had to learn about it.