Does masturbation always have to be private?
Not necessarily! Two or more people masturbating in each other's presence, also known as mutual masturbation, is perfectly fine as long as consent has been received by everyone involved. Keep in mind that mutual masturbation should still be a private act in the sense that no one who has not consented should be forced to watch.
Can you get STIs from giving a hand job or fingering someone?
A hand job is when a person manually stimulates the penis. Fingering is when someone inserts their finger/s into either the vagina or anus to manually stimulate the area. These activities are safe and healthy; however, it's important to wash your hands beforehand and make sure that no sexual fluids are on the hand/fingers. Otherwise, there would be a risk of STI transmission and, in rare cases, a risk of pregnancy (if someone had ejaculate on their finger and inserted it into another person's vagina).