L'estoreta és dins la tenda de campanya.

Because the feminine singular article la is shortened before a word that begins with a vowel, becoming l'.

So:

• la + estoreta → l'estoreta

This is called elision. The same thing also happens with el, so l' does not tell you by itself whether the noun is masculine or feminine.

The ending -eta is a very common diminutive ending in Catalan.

• estora = mat
• estoreta = little mat / small mat

A diminutive can show:

• small size
• affection
• informality
• sometimes just the usual everyday word in context

So estoreta often feels a bit like little mat rather than just mat.

The accent in és is important because it distinguishes it from es.

• és = is (from the verb ser)
• es = a pronoun, as in reflexive verbs

So the accent marks that this is the verb form is.

In Catalan, ser is very commonly used to say where someone or something is in a simple, neutral way.

So:

• L'estoreta és dins la tenda de campanya is perfectly natural.

Many learners expect estar because of Spanish, but Catalan does not always work the same way. In everyday Catalan, you may also hear està in some contexts or varieties, but és is very standard here for simple location.

Here dins means inside.

With a following noun phrase, Catalan very often uses:

• dins de la tenda de campanya
• a dins de la tenda de campanya

You may also hear the shorter form used as in your sentence:

• dins la tenda de campanya

So the main idea is the same: inside the tent.
If there is no noun after it, a dins is especially common:

• És a dins = It is inside

Yes. Tenda de campanya is a normal and very common Catalan expression for a camping tent or tent.

Sometimes people may just say tenda if the context is clear, but tenda de campanya is the full expression.

A useful note for English speakers: do not assume Catalan tenda works exactly like Spanish tienda. In standard Catalan, shop/store is usually botiga, not tenda.

Because de campanya is functioning as a descriptive phrase, almost like camping or for camp.

So:

• tenda de campanya = camping tent / tent

If you added an article:

• tenda de la campanya

that would usually mean something different, more like the tent of the campaign.

So in your sentence, de campanya is just part of the fixed noun phrase.

The ny in campanya is pronounced like the ny in canyon.

So:

• campanya ≈ cam-PA-nya

A few stress points in the full sentence:

• estoreta → stress on re
• és → stressed
• tenda → stress on ten
• campanya → stress on pa

So the sentence is roughly pronounced like:

• l-es-to-RE-ta ez dins la TEN-da de cam-PA-nya

The exact vowels depend on accent and dialect, but the key sound for learners is the ny in campanya.