La sopa se calienta rápido en el microondas.

Because calentarse is commonly used to mean to get hot / to heat up.

So La sopa se calienta rápido means something like:

• The soup heats up quickly
• The soup gets hot quickly

Here, se is not saying that the soup is literally doing the action to itself in a deliberate way. It often gives the idea of a change of state: the soup goes from not hot to hot.

Compare:

• Caliento la sopa. = I heat the soup.
• La sopa se calienta rápido. = The soup heats up quickly.

Not in the most literal English-style sense of it heats itself.

In beginner courses, forms like this are often introduced as reflexive, because they use se, but in real usage this is often better understood as a pronominal verb or a verb describing a process/change:

• La puerta se abre. = The door opens.
• La sopa se calienta. = The soup heats up.

So although it looks reflexive in form, the most natural translation is usually not the soup heats itself, but the soup heats up.

It most naturally means the soup heats up quickly in the microwave.

The focus is on what happens to the soup, not on who does it.

If you wanted to say you heat the soup quickly, Spanish would more likely use an explicit subject or object structure, for example:

• Calientas la sopa rápido en el microondas. = You heat the soup quickly in the microwave.
• Se calienta la sopa rápido en el microondas. could exist in some contexts, but that structure would be interpreted differently and is less straightforward for a learner.

So in your sentence, la sopa is the thing undergoing the change.

Because in everyday Spanish, especially in speech, an adjective like rápido is very often used adverbially.

So both of these are possible:

• La sopa se calienta rápido.
• La sopa se calienta rápidamente.

Both mean the same thing, but there is a slight difference in feel:

• rápido = more common, more natural in everyday conversation
• rápidamente = a bit more formal or heavier in style

This is very normal in Spanish:

• Habla claro. = He speaks clearly.
• Corre rápido. = She runs quickly.

Because en is used to mean in or inside the microwave.

• en el microondas = in the microwave

Using a would usually suggest movement to something, which does not fit here.

Compare:

• Pongo la sopa en el microondas. = I put the soup in the microwave.
• La sopa se calienta en el microondas. = The soup heats up in the microwave.

So en is the normal preposition here.

Good question. Microondas is one of those words that can confuse learners.

(Horno) microondas originally refers to microwave oven, and the full expression is masculine because the implied noun is horno:

• el horno microondas → shortened to el microondas

So even though ondas is plural in form, the everyday word is:

• el microondas = the microwave

In Spain, microondas is the standard word for the appliance.

Because Spanish often puts the thing being talked about first, especially when that thing is the topic of the sentence.

Here the topic is the soup, and then the sentence tells us something about it:

• La sopa → what are we saying about it?
• se calienta rápido en el microondas → it heats up quickly in the microwave

This is very natural word order in Spanish.

You could change the order in some contexts, but the original version is the most neutral and natural.

Spanish uses definite articles much more often than English.

So la sopa can mean:

• the soup
• or sometimes soup in a general/contextual sense, depending on the situation

In a sentence like this, Spanish normally wants the article:

• La sopa se calienta rápido...

Leaving it out would usually sound incomplete or unnatural unless you were using a different structure.

Yes, but it means something different.

• La sopa se calienta rápido. = The soup heats up quickly.
  This describes a process.
• La sopa está caliente. = The soup is hot.
  This describes a state/result.

So:

• se calienta = becoming hot
• está caliente = already hot

That difference is important.

Not usually if you mean quickly.

• rápido = quickly / fast
• pronto = soon

So:

• La sopa se calienta rápido. = it becomes hot fast
• La sopa se calienta pronto. = it heats up soon / before long

Those are related ideas, but not the same.

It is used broadly across the Spanish-speaking world, not only in Spain. In Spain, el microondas is completely standard.

In casual speech in Spain, some people may also shorten it to:

• el micro

For example:

• Mete la sopa en el micro. = Put the soup in the microwave.

But for learners, el microondas is the safest full form to use.

Not really as the main meaning.

Spanish se can sometimes create passive-like sentences, but here the most natural reading is not a true passive. It is more like:

• The soup heats up quickly in the microwave

A more clearly passive-like idea would usually involve an agent or a different structure, for example:

• La sopa es calentada... — grammatically possible, but usually unnatural in everyday Spanish
• La sopa se calienta... — much more natural, and usually understood as a process/change

So for a learner, the best interpretation here is heats up, not is heated.