arm
膊 centers on the arm as a body part. It is a specific anatomical term for the arm, without broad abstract extensions.
膊 combines ⺼ (flesh/body part) with 尃, which likely contributes the sound. The character is used specifically for the arm.
The left side ⺼ is flesh, and the right side 尃 looks like a spread-out hand with a measuring tool. Picture a fleshy arm spreading out to measure something.
For ハク, imagine a strong arm hacking through a thick branch: hack -> ハク, and the arm does the work.