Slide it under their neck, and hold their upper body with it. You’re supporting their head (which is comfy for them), and you don’t need to do gymnastics or stuff it under their torso.
I’m not sure which hand you are referring to, as both of my hands are around my partner.
When I spoon Mrs. Stone, and I am laying on my left side, my left arm is straight out in front of me in front of the next pillow. She lies down with her back to me and her head on the pillow, such that my biceps is in the crook of her neck. This allows my forarm to bend toward her so my left hand can grip her… stomach. My right arm reaches over and around her allowing my right hand to stroke her… hair.
It’s like a hug from behind but while lying down if you do it right.
Either under the pillow, under both our heads, elbow bent so my hand is under my own head, or under her neck, holding her left (upper) shoulder (I always spoon whilst on my right side, as my left shoulder is wonky and doesn’t like being on the bottom).
I sleep on my side, spooned up to my husband while he is on his back. My head on his chest. Consequently, I have the arm problem, which goes straight down my side and usually with my hand on his leg. Make sense?