Na końcu koncertu publiczność wstaje i bije brawo.

Because na końcu means at the end.

• na końcu koncertu = at the end of the concert
• w końcu = finally / eventually

So here na końcu is the correct choice. W końcu koncertu would not mean the same thing and sounds wrong in this context.

Koncertu is genitive singular of koncert.

The phrase na końcu czegoś uses genitive for the thing whose end you mean:

• na końcu filmu = at the end of the film
• na końcu dnia = at the end of the day
• na końcu koncertu = at the end of the concert

So literally, it is at the end of the concert.

Końcu is locative singular of koniec.

After na, when you mean a location or position, Polish often uses the locative:

• na stole
• na końcu

So in na końcu koncertu:

• końcu = locative
• koncertu = genitive dependent on koniec

Because publiczność is a collective noun. It refers to a group of people, but grammatically it is singular.

That is why the verb is singular:

• publiczność wstaje = the audience stands up

Not:

• publiczność wstają

Also, publiczność is grammatically feminine, so in the past tense you would say:

• publiczność wstała
• publiczność była zachwycona

The infinitive is wstawać.

This is the imperfective verb. Its perfective partner is wstać.

• wstawać = to be getting up / to get up habitually
• wstać = to stand up once, as a completed action

So:

• wstaje comes from wstawać
• wstanie comes from wstać

Because wstaje is a present-tense imperfective form, while wstanie is a future perfective form.

In this sentence, Polish is describing what happens as a scene or what typically happens:

• publiczność wstaje i bije brawo

That sounds like:

• a general description
• a live narration
• or a vivid present-time scene

If you said wstanie, it would mean will stand up, so the time reference would change.

Polish often uses the present tense to describe:

1. something that usually happens
2. something happening right now
3. a vivid, story-like description

So Na końcu koncertu publiczność wstaje i bije brawo can mean something like:

• At the end of a concert, the audience stands up and applauds
• or a vivid the audience stands up and applauds

If you wanted one finished past event, you would more likely say something like:

• Na końcu koncertu publiczność wstała i zaczęła bić brawo.

It means applauds or claps.

This is a fixed expression:

• bić brawo = to applaud

Even though bić usually means to beat / hit, in this expression it is idiomatic. So you should learn bić brawo as a whole chunk.

A close one-word alternative is:

• klaskać = to clap

In this expression, brawo is usually treated as a fixed, indeclinable word.

So learners should remember the phrase as:

• bić brawo

You may also encounter bić brawa, but bić brawo is very common and is the easiest fixed form to learn first.

Not exactly.

• na końcu koncertu = at the end of the concert, at the final part
• pod koniec koncertu = toward the end of the concert, near the end

So na końcu sounds more final and exact.

Yes. Polish word order is fairly flexible.

This sentence is neutral and natural:

• Na końcu koncertu publiczność wstaje i bije brawo.

Starting with Na końcu koncertu sets the time frame first. That is very common.

You can move parts around for emphasis, but not every version sounds equally natural. For example:

• Publiczność wstaje i bije brawo na końcu koncertu is possible, but it puts the time phrase later and changes the focus.

So the original order is a very good default.

Because Polish has no articles.

So publiczność can mean:

• the audience
• an audience

And koncertu can mean:

• of the concert
• of a concert

The exact meaning comes from context, not from words like the or a.