Yahweh's "arm" represents his ability and power. Alternate translation: "Yahweh used his own power to save the people" (See: [[rc://en/ta/man/jit/figs-metonymy]])
"Righteousness" here is a quality that acts like a person. The word can be translated as an adjective. Alternate translation: "he did right as he always would do" (See: [[rc://en/ta/man/jit/figs-personification]] and [[rc://en/ta/man/jit/figs-abstractnouns]])