ברגע שאני מגיעה הביתה, אני מחממת את המרק במיקרוגל.

Because Hebrew present-tense verb forms agree with gender and number, even with אני.

So a female speaker says:

• אני מגיעה
• אני מחממת

A male speaker would say:

• אני מגיע
• אני מחמם

So the sentence as written is spoken by a woman/girl. In Hebrew present tense, אני does not remove the need to choose masculine or feminine.

As written, it most naturally sounds like a habitual/repeated action:

• Whenever / as soon as I get home, I heat the soup in the microwave.

Hebrew often uses the present tense for routines and general behavior.

If you want to talk about one specific future occasion, Hebrew would usually prefer the future tense:

• ברגע שאגיע הביתה, אחמם את המרק במיקרוגל.

So:

• present tense = habit, routine, general pattern
• future tense = one specific future event

ברגע ש־ means the moment that, as soon as, or sometimes simply when.

So:

• ברגע = at the moment / the instant
• ש־ = a linking word meaning something like that/when

In this sentence, ש־ is attached directly to אני, so you get:

• ברגע שאני = the moment I... / as soon as I...

This is a very common Hebrew structure.

Because ש־ is a prefix in Hebrew. It attaches to the following word.

So:

• ש + אני = שאני
• ש + הוא = שהוא
• ש + היא = שהיא

This is normal Hebrew spelling. The ש־ here is not written separately.

הביתה is a special, very common form meaning homeward / to home / home with movement toward home.

In Hebrew, some place words can take a directional ending ־ה, which gives the sense of toward that place. So:

• בבית / בביתי = at home / in my home
• הביתה = home / to home

Compare:

• אני בבית = I am at home
• אני הולכת הביתה = I am going home

So in your sentence, מגיעה הביתה means arrive home / get home.

את is the Hebrew marker for a definite direct object.

It appears before a direct object that is definite, such as:

• a noun with ה־
• a proper name
• a pronoun

Here, המרק means the soup, which is definite, so Hebrew uses את:

• אני מחממת את המרק

If the object were indefinite, you would usually leave את out:

• אני מחממת מרק = I heat/warm soup

So את does not mean a separate English word here; it is a grammar marker.

In this sentence, במיקרוגל means the natural Hebrew equivalent of in the microwave or using the microwave.

The preposition ב־ often covers meanings that in English may be translated as:

• in
• at
• sometimes with/by means of

So לחמם במיקרוגל is the normal way to say heat in the microwave.

Usually, yes—it is very natural to repeat it here.

One reason is that Hebrew present-tense forms do not clearly show person the way past and future forms do. For example, מחממת could mean:

• I heat (female speaker)
• you heat (to a female)
• she heats

So adding אני makes the subject clear.

You may sometimes hear the second אני omitted in casual speech if the meaning is obvious:

• ברגע שאני מגיעה הביתה, מחממת את המרק במיקרוגל.

But the full version with both אני is very normal and clear.

Because מחממת means heat something, while מתחממת means warm up / become warm.

Here the speaker is doing the action to the soup, so Hebrew uses the transitive verb:

• לחמם = to heat, to warm (something)
• אני מחממת את המרק = I heat the soup

But:

• להתחמם = to warm up / get warm
• המרק מתחמם במיקרוגל = The soup is warming up in the microwave

So the sentence needs מחממת because there is a direct object: את המרק.

Yes. That would be grammatical, but the nuance changes a little.

• כשאני מגיעה הביתה = when / whenever I get home
• ברגע שאני מגיעה הביתה = the moment / as soon as I get home

So ברגע שאני feels more immediate and a bit more emphatic. It suggests that the soup-heating happens right away.

If you want a more neutral when, then כשאני works well.