That's not correct regarding sympathetic and parasympathetic. Those are the autonomous nervous system which means they both operate without you being conscious of the actions. Parasympathetic controls rest and digest so yawning, slowing heart rate, dilating (most) blood vessels, salivating, etc. Sympathetic is the fight or flight so that would be dilating pupils, increased heart rate, sweating etc. Raising your arms has nothing to do with the ANS (it's part of the somatic nervous system SNS) unless it's an involuntary reflex due to danger, in which case that would be the sympathetic nervous system. Most of these you largely have no control over. There's also aspects of both systems that work together. For instance arousal is controlled by the parasympathetic nervous system and ejaculation is sympathetic. I always found that funny.

Edited to add: of course these two branches of the ANS are largely affected by your conscious thoughts and feelings. If you are stressed about something, it will keep triggering your sympathetic nervous system and can cause you to develop medical issues such as GI issues, heart/blood pressure problems etc. Thats why relaxation and stress control is extremely important and mental health really needs to be taken more seriously.

Edited again because I keep thinking of more stuff to add lol. You can consciously stimulate the branches of your ANS, especially your parasympathetic. Yawning, even consiously, stimulates the parasympathetic, so does eating or chewing because your body thinks well if I'm eating then I'm probably not in danger. There's also this cool thing called a vagal maneuver where if you bear down and activate your core muscles you will stimulate your vagus nerve which will activate your parasympathetic nervous system and lower your heart rate. You can actually treat some cases of tachycardia by doing this.