Una auxiliar de vuelo me ayudó a guardar la mochila y otra me trajo agua.

Because auxiliar is a common-gender noun in Spanish: the noun itself does not change, and the article or other words around it show the gender.

• una auxiliar de vuelo = a female flight attendant
• un auxiliar de vuelo = a male flight attendant

Then otra is feminine too, so it tells you the second person is also female. If both were male, you would have otro instead.

Me is the object pronoun meaning to me or for me, depending on the verb.

• me ayudó a guardar la mochila = she helped me put away the backpack
• me trajo agua = she brought water to me / for me

Spanish uses these short object pronouns very often, where English may simply rely on word order.

Because ayudar is commonly followed by a + infinitive when you say that someone helped do an action.

So:

• ayudar a guardar = to help put away
• ayudar a subir = to help get on / go up
• ayudar a encontrar = to help find

So me ayudó a guardar la mochila literally has the structure she helped me to put away the backpack.

Here guardar means to put away, store, stow, keep.

In a flight context, guardar la mochila usually means putting the backpack in the overhead bin or under the seat. So it is not save in the computer-file sense here.

Spanish often uses the definite article instead of a possessive when the owner is already obvious from context.

Here, it is already clear that the backpack belongs to the speaker, so la mochila sounds natural. Spanish does this more often than English.

• me ayudó a guardar la mochila = natural
• me ayudó a guardar mi mochila = also possible, but more explicit

Using mi mochila would add a bit more emphasis to whose backpack it was.

Because Spanish often leaves out a noun when it has just been mentioned and is still clear.

So:

• otra = another one / another female one
• full version: otra auxiliar de vuelo

The full version is grammatically fine, but repeating auxiliar de vuelo would sound less natural and more repetitive.

There is a difference:

• otra = another one
• la otra = the other one

In this sentence, otra simply introduces a second flight attendant. It does not strongly emphasize that she was the only other one.

If you said la otra, it would sound more like you are specifically identifying the remaining one out of two.

Because the sentence describes completed past actions:

• one flight attendant helped
• another brought water

These are seen as finished events in a sequence, so the preterite is the normal tense.

If the imperfect were used instead, it would suggest background information, repetition, or an ongoing situation rather than two completed actions.

Yes. Traer has an irregular preterite stem:

• traje
• trajiste
• trajo
• trajimos
• trajisteis
• trajeron

So trajo is the normal third-person singular preterite form of traer.

Because agua here is being used as an uncountable substance in a general, indefinite way.

• me trajo agua = she brought me water / some water

That is very natural in Spanish. If you used an article, the meaning would change:

• me trajo el agua = she brought me the water
• me trajo un agua = she brought me a water, often meaning a serving, bottle, or glass in context

Because agua is a feminine noun, but in the singular it takes el before a stressed a- sound for ease of pronunciation:

• el agua fría
• el águila blanca

Notice that adjectives still stay feminine:

• el agua fría, not el agua frío

In the plural, it goes back to the normal feminine article:

• las aguas

Yes. Auxiliar de vuelo is a normal and widely understood term in Spain.

You may also hear:

• azafata for a female flight attendant
• azafato for a male flight attendant
• tripulante de cabina as a more formal or technical term

But auxiliar de vuelo is perfectly standard and clear.