"Seldom" is from Old English (compare to High German "selten"). "Rarely" is from French / Latin (compare French "rarement"). They both mean the same thing, but in conversation we usually just say "not often", for example: "He doesn't study too often."

While both can be interchangeable, I believe a lot of people will say "rarely" means lesser in frequency than "seldom"

Same. Not happening very often. Context might help.

They are interchangeable I think. Maybe someone can come up with a counterexample. "Rarely" may be stronger than "seldom", ie, it may connote an occurrence of lower frequency.