Learn Hardcore Portuguese (Portugal): Durante o intervalo, a bateria faz um solo e o estádio inteiro aplaude. - During the break, the drums play a solo and the whole stadium applauds.

Durante o intervalo = during the break, focusing on the whole duration of that time. It suggests something that happens at some point (or throughout) while the break is going on.
No intervalo = in/at the break, more like saying something happens at that time, but it doesn’t explicitly emphasise the duration.

In this sentence, Durante o intervalo is very natural because a solo is performed during that break period. No intervalo would not be wrong, but it sounds a bit more like a time slot, not so much the span of time.

What does bateria mean here? Is it “battery”, “band”, or “drums”?

In this context, a bateria refers to the drums / drum kit or possibly the drum section of a band, not an electrical battery.

Some uses of bateria:

Electrical battery: uma bateria de telemóvel (a phone battery)
Musical instrument: a bateria = the drum kit
Group of drummers (especially in Brazilian samba schools): a bateria da escola de samba

In European Portuguese, at a stadium context, a bateria is most naturally understood as the drums / the drummer(s), i.e. the percussion playing a solo.

Why is the verb faz (from fazer) used with um solo? Could I say toca um solo instead?

Both are possible, but they have slightly different flavours:

fazer um solo = literally to do a solo, very common when talking about a musician performing a solo section.
tocar um solo = to play a solo, also correct and clear.

In everyday speech, fazer um solo is very idiomatic for musicians, especially with instruments. So:

A bateria faz um solo.
A guitarra faz um solo.
O saxofone faz um solo.

Tocar is more directly tied to playing an instrument, so tocar um solo is also fine and might sound a bit more “schoolbook correct,” but fazer um solo is extremely natural.

Why is it um solo and not um solo de bateria? How do we know which instrument is doing the solo?

We know from context: the subject a bateria comes right before faz um solo, so it’s understood that it’s a drum solo.

If you needed to be explicit, you could say:

Durante o intervalo, a bateria faz um solo de bateria – but this sounds redundant.
Better: Durante o intervalo, o baterista faz um solo de bateria.

In normal usage, when the instrument (or player) is clear, you simply say fazer um solo, and listeners infer the instrument.

Why does the verb aplaude appear in the singular when it refers to the whole stadium (many people)?

Because the grammatical subject is o estádio inteiro, which is singular:

o estádio = singular noun (the stadium)
inteiro = singular adjective agreeing with estádio
So the verb must be singular: o estádio inteiro aplaude

Semantically, we know that many people are applauding, but grammar follows the form of the subject, not the real-world number of individuals.

If you wanted a clearly plural subject, you could say:

As pessoas no estádio aplaudem. – The people in the stadium applaud.
Os adeptos aplaudem. – The fans applaud.

Could I say o estádio inteiro aplaudem because many people are clapping?

No, that would be considered incorrect in standard Portuguese.

o estádio inteiro is grammatically singular, so the verb must also be singular: aplaude.
aplaudem would need a plural subject, for example:
- Todos no estádio aplaudem.
- Os adeptos no estádio aplaudem.

Even though you’re thinking of lots of people, the verb form has to follow the singular noun o estádio.

What is the difference between o estádio inteiro and todo o estádio?

They are very close in meaning and both are idiomatic:

o estádio inteiro = literally the entire stadium
todo o estádio = literally all the stadium / the whole stadium

In this sentence, both sound natural:

…e o estádio inteiro aplaude.
…e todo o estádio aplaude.

If anything, todo o estádio is slightly more neutral and very common; o estádio inteiro can sound a bit more vivid or emphatic (like the stadium as a whole), but the difference is minor.

Why is there a comma after Durante o intervalo?

Durante o intervalo is an adverbial phrase of time placed at the beginning of the sentence. In Portuguese, when you front an adverbial like this, it is very common (and usually recommended) to separate it with a comma:

Durante o intervalo, a bateria faz um solo…
À noite, eu estudo.
No verão, vamos à praia.

If you moved the time phrase to the end, you would normally not use a comma:

A bateria faz um solo durante o intervalo.

Could this sentence also use the past tense, like fez um solo and aplaudiu? How would that change the meaning?

Yes, you can put it in the past:

Durante o intervalo, a bateria fez um solo e o estádio inteiro aplaudiu.

Differences:

faz / aplaude (present) can be:
- describing something that happens regularly (e.g. every match), or
- a “historic present” used for vivid narration of a specific episode.
fez / aplaudiu (past perfect / preterite) clearly describes a completed event in the past.

So use the present for general routines or lively storytelling, and the past for a specific past event that you’re reporting more neutrally.

Is the word order fixed? Could I say A bateria faz um solo durante o intervalo e o estádio inteiro aplaude?

Yes, that’s also correct and natural:

A bateria faz um solo durante o intervalo e o estádio inteiro aplaude.

The differences:

Durante o intervalo, a bateria faz um solo…
Starts by setting the time frame; stylistically, it makes the timing more prominent.
A bateria faz um solo durante o intervalo…
Starts with the subject; more neutral or “plain” word order.

In Portuguese, moving adverbial phrases like durante o intervalo is quite flexible, as long as the result is clear and not ambiguous.