El concurso de preguntas es difícil; eso sí, hacemos pleno.

Yes. Concurso de preguntas is understandable and natural for a contest where people answer questions.

A few notes:

• concurso = contest / competition
• de preguntas = of questions / question-based

Depending on context, Spanish speakers might also say:

• concurso on its own, if the context is already clear
• concurso de preguntas y respuestas for question-and-answer contest
• quiz, especially in some modern or informal contexts

So concurso de preguntas works well, especially if you want to be explicit.

Because de + noun is often used in Spanish to describe the type or purpose of something.

So:

• concurso de preguntas = a question contest / quiz contest

Here, preguntas is naming the category of contest, not referring to specific questions already mentioned.

Compare:

• tienda de ropa = clothes shop
• juego de mesa = board game
• concurso de preguntas = quiz contest

If you said concurso de las preguntas, it would sound like the contest of the questions, as if you were referring to some specific questions already known, which is not the idea here.

Both ser and estar can sometimes be used with difícil, but the nuance changes.

Here, es difícil presents the contest as difficult by nature or in general:

• El concurso es difícil = the contest is difficult

If you said está difícil, it would sound more like:

• it feels difficult right now
• it is turning out to be difficult
• it is difficult in the current situation

So es difícil is the more neutral, general description.

Eso sí is a very common expression meaning something like:

• that said
• mind you
• however
• still
• though

It introduces a qualification or contrast.

So the logic is:

• the contest is difficult;
• that said / even so, we get everything right

It does not mean a literal that yes in normal translation. It functions as a fixed discourse marker.

It is best learned as a fixed expression.

Literally, it is made of:

• eso = that
• sí = yes

But in real usage, Spanish speakers use eso sí as an idiomatic connector, not as a word-for-word phrase.

It often appears with pauses in speech and commas in writing:

• Es caro; eso sí, es buenísimo.
• No tiene mucho tiempo. Eso sí, cuando viene, ayuda muchísimo.

So it is better to think of it as a chunk meaning that said / mind you / however.

Hacer pleno is an idiomatic expression meaning:

• to get everything right
• to score full marks
• to have a perfect score
• to make a clean sweep

Here, pleno does not simply mean full in the ordinary adjective sense. In this expression, it refers to getting the maximum possible result.

So in a quiz context, hacemos pleno means that all the answers are correct.

Because hacer pleno is a set collocation: a normal verb-noun combination that Spanish speakers just use that way.

This is very common in Spanish. Often, the verb is not the one an English speaker would expect if translating literally.

For example, Spanish says:

• hacer una pregunta = ask a question
• hacer fotos = take photos
• hacer pleno = get everything right / achieve full marks

So even though hacer usually means to do or to make, in many fixed expressions it has to be learned together with the noun.

Because Spanish usually leaves out subject pronouns when the verb ending already shows who the subject is.

• hacemos = we do / we make

The ending -emos clearly tells you it is nosotros.

So:

• hacemos pleno
  and
• nosotros hacemos pleno

both mean the same thing, but the version without nosotros is more normal unless you want emphasis or contrast.

For example:

• Ellos fallan; nosotros hacemos pleno.
  Here nosotros is included for contrast.

The present tense in Spanish can do several jobs, depending on context. Here it could mean:

• a general/habitual idea: we get full marks
• a present situation: we’re getting full marks
• a vivid narrative style: we get a perfect score

If the speaker were talking about one completed event in the past, Spanish would more likely say:

• hicimos pleno

So hacemos is perfectly normal, but its exact time reference depends on the wider context.

The semicolon links two closely related complete clauses:

• El concurso de preguntas es difícil
• eso sí, hacemos pleno

It creates a stronger pause than a comma, but a weaker break than a full stop.

It works well here because the second part comments on or qualifies the first.

You could also see other punctuation in real Spanish, such as:

• El concurso de preguntas es difícil. Eso sí, hacemos pleno.
• El concurso de preguntas es difícil, eso sí, hacemos pleno.

But the semicolon is a neat, clear choice for this kind of contrast.

Yes. Pleno can mean different things depending on context.

Common meanings include:

• full / complete as an adjective
• a plenary session or official meeting, especially in politics or local government
• in certain games, competitions, quizzes, or betting contexts, a perfect result

In this sentence, it is that last meaning, inside the expression hacer pleno. So you should understand it from the whole phrase, not as an isolated word.