En la barbacoa, el carbón está en brasas bajo la llama.

It can mean either, depending on context.

Here, la barbacoa most naturally refers to the grill / barbecue setup itself, because the sentence is talking about where the charcoal is. In other contexts, una barbacoa can also mean a barbecue gathering or meal.

Spanish often uses en for something located in / on / at a space or cooking device, especially when it is thought of as an area or structure.

• en la barbacoa = in/on the barbecue grill
• sobre la barbacoa would sound more like on top of the outside surface of the barbecue

So en la barbacoa is the natural choice if the charcoal is inside the grill area.

Spanish uses the definite article much more often than English does.

Here, el carbón can mean:

• the charcoal that is in the barbecue, or
• charcoal as the relevant substance being discussed in this situation

English often drops the article in similar cases, but Spanish usually keeps it.

No.

• carbón = coal or charcoal
• carbono = carbon, the chemical element

So in a barbecue context, carbón is the right word. Also notice the accent: carbón is stressed on the last syllable.

Because estar is used for location and for many temporary states or conditions.

In this sentence, the charcoal:

• is located somewhere: en la barbacoa, bajo la llama
• is in a certain burning state: en brasas

So está is the correct verb. Es would sound wrong here.

It describes the charcoal as being in the state of glowing embers or reduced to embers.

The noun brasa means ember. In this phrase, en brasas works like a state expression, similar to other Spanish patterns such as:

• en llamas
• en ruinas
• en cenizas

So it is not just naming an object; it is describing the condition the charcoal is in.

Because embers are normally thought of as many glowing pieces, not just one.

• una brasa = one ember
• brasas = embers

So en brasas is the natural form if you are talking about charcoal burning as glowing coals.

Both can mean under / beneath the flame.

The difference is mostly stylistic:

• bajo = shorter, slightly more compact, sometimes a bit more formal or literary
• debajo de = very common in everyday speech

So bajo la llama is perfectly understandable, but debajo de la llama would also be normal.

Yes, las llamas is also possible, depending on the image you want.

• la llama suggests the flame as one visible body of fire
• las llamas suggests the flames more generally or several tongues of fire

So the singular is not wrong; it just presents the fire as a single flame.

It is understandable, and the grammar is clear. A native speaker might still choose slightly different wording depending on the situation.

For example, someone might prefer:

• debajo de la llama instead of bajo la llama
• las llamas instead of la llama if there is more visible fire

But the original sentence is still a useful and understandable Spanish sentence.