What do non-combatants do when under attack or an ambush?

Their job.

Non-combatants has a specific meaning: a person who is not engaged in fighting during a war, especially a civilian, chaplain, or medical practitioner; also those former combatants who are incapable of resistance because of injuries or choice.

I assume you are talking about non-combat arms MOS's. With very few exceptions, you pick up the weapon you were issued and think of the training you were given and you fight. Some MOS's like Corpsmen/medics and Chaplains tend to the wounded and dying. If you curl up in the fetal position, soil your undies, and refuse to fight, or pack it in and head for the rear without receiving orders to do so, better hope I die in the fight. Article 99 UCMJ.

First and foremost, Commanders determine when and how those non-combatants in their area of operations, is those the theater commander has allowed to accompany troops, are exposed to risk. Others entering the combat zone on their own can only expect care and protection on an in extremis basis.

the Commander assigns a non-Combatant to a group of soldiers while on a mission outside the wire. There has to be a task and purpose to their presence other than tactical tourism. I saw an incident in 2011 where a female civilian advisor and a law enforcement advisor were out on a patrol, dismounted in a crowded market, when an insurgent doused her with an accelerant and emoliated her. Was there anything that could have been done? Unlikely. The patrol was with her and had her teamed with someone who was armed. In the blink of an eye, it was done.

Normally, the subordinate commander or person in charge (like a SSG leading a patrol) spends time upfront rehearsing what to do, where to go. Tactics, techniques and procedures vary based on ROE, environment, mission, and threat. You can get a sense from Sebastian Junger's "War". I was assigned to,provide a personal protection detail to a DA Civilian. I used my Company HQ as a PErsonal Security Detachment and we took her where she needed to go and covered her to ensure she was safe. We rehearsed react to contact drills and bump drills between vehicles among many things. Stuff was straight among the trigger pullers before we included her.

Official imbedded media underwent media preparation by the US military to teach them as best they could for reaction drills, IMT, etc. the government civilians usually go through theater specific training. The key is integrating them in the unit SOP when they show up.

I can't get in too much more detail due to OPSEC. If you inherit one on a deployment, assume they know nothing and integrate them. They can render aid. They can observe areas in and around the vehicle, let them know what threats look like. Dismounting vehicle drills. Roll over drills. Helping with vehicle house keeping and tie downs. A .50 caliber can can knock their block off just like yours. They should not be handling weapons/munitions unless it is covered in the established ROE and they are trained on it.