Learn Italian: Il superstite viene soccorso e portato sulla barella. - The survivor is rescued and carried on the stretcher.

The verb venire is used for the dynamic passive voice, which focuses on an action happening in the moment (the process of being rescued). If we used rimane or resta, it would focus on a resulting state, which doesn't make sense for these specific actions. The resultative passive is better for lasting states, like è rimasto ferito (he was left injured).

Is the word superstite exclusively masculine?

No, it can be either masculine or feminine. Like many Italian words denoting people that end in -e (or -ista), the noun itself stays the same in the singular, and you just change the article to match the person's gender. A female survivor would be la superstite.

What is the infinitive of soccorso, and how do I remember its irregular past participle?

The infinitive is soccorrere (to help/rescue). It is a compound of a prefix and correre (to run), literally meaning "to run to someone's aid". Because it is based on correre (which has the past participle corso), its past participle follows the exact same pattern: soccorso.

Why doesn't portato have an auxiliary verb like viene or è in front of it?