Aquesta actriu i aquest actor treballen junts amb una orquestra molt coneguda.

Because aquesta and aquest are the feminine and masculine singular forms of this.

• aquesta actriu = this actress
• aquest actor = this actor

In Catalan, demonstratives usually agree in gender and number with the noun they go with.

In standard Catalan, the usual feminine form of actor is actriu.

So:

• actor = male actor
• actriu = female actor / actress

This is just the standard noun pair you need to learn. It does not form the feminine in the same way English learners might expect from Spanish.

Because aquesta and aquest already function as determiners, like this in English.

So Catalan normally says:

• aquesta actriu
• aquest actor

not:

• la aquesta actriu
• el aquest actor

This works the same way as English: you say this actress, not the this actress.

Treballen is the 3rd person plural present form of treballar (to work).

The subject is aquesta actriu i aquest actor, which is two people joined by i (and), so the verb must be plural.

• treballa = he/she works
• treballen = they work

Junts means together here.

It shows that the actress and the actor work together, not separately. Catalan often uses a form that looks like an adjective for this idea, and it agrees with the people involved.

So in this sentence, junts adds the idea of joint action:

• treballen junts = they work together

Because the group includes one woman and one man, and in Catalan a mixed-gender plural normally takes the masculine plural form.

So:

• two women: juntes
• two men: junts
• one woman + one man: junts

This is a very common agreement pattern in Catalan.

Amb means with.

In this sentence it introduces the person or group they are working with:

• treballen ... amb una orquestra = they work ... with an orchestra

It is the normal Catalan preposition for with in this kind of context.

Because the first two nouns are introduced by demonstratives:

• aquesta actriu
• aquest actor

But orquestra is not introduced by a demonstrative, so it needs an article:

• una orquestra

So the pattern is:

• demonstrative + noun: aquesta actriu
• article + noun: una orquestra

Because molt here is an adverb meaning very, not an adjective meaning much/many.

As an adverb, molt does not change for gender or number:

• molt coneguda = very well-known
• molt conegut = very well-known
• molt conegudes = very well-known

The word that changes is coneguda, because it agrees with orquestra, which is feminine singular.

Because in Catalan, adjectives often come after the noun.

So:

• una orquestra coneguda
• una orquestra molt coneguda

This is the most neutral and natural order here. English often puts adjectives before the noun, but Catalan frequently places them after it.

Coneguda literally means known, but in this context it is best understood as well-known or famous/recognized.

So una orquestra molt coneguda means an orchestra that many people know about, not an orchestra that the speakers personally know.

Yes. I is the normal Catalan word for and.

Here it simply joins the two parts of the subject:

• aquesta actriu i aquest actor

One useful difference from Spanish is that Catalan keeps i even before words beginning with an i sound; it does not change form.

A little, yes, but the original order is the most neutral and natural.

Aquesta actriu i aquest actor treballen junts amb una orquestra molt coneguda is a straightforward standard sentence.

Catalan does allow some movement for emphasis, but if you are learning, this sentence is a very good model to follow:

• subject
• verb
• junts
• prepositional phrase

So the given order is the best one to treat as your default pattern.