Mi amiga es modesta y solidaria; por eso todo el mundo se fía de ella.

Because Spanish uses the short possessive adjective before the noun:

• mi amiga = my friend
• tu casa = your house

The longer forms like mío / mía / míos / mías are usually used after a verb or on their own:

• La amiga es mía = The friend is mine

So before a noun, you say mi amiga, not mía amiga.

Spanish usually uses ser for inherent or characteristic qualities, and estar for states or conditions.

Here, modesta and solidaria describe what kind of person she is, so ser is the natural choice:

• Es modesta = she is modest
• Es solidaria = she is supportive / caring / shows solidarity

Using estar here would sound unusual because it would suggest a temporary state rather than a personal trait.

Because they agree with amiga, which is a feminine singular noun.

In Spanish, adjectives normally agree in gender and number with the noun they describe:

• amigo modesto y solidario = masculine singular
• amiga modesta y solidaria = feminine singular
• amigas modestas y solidarias = feminine plural

That is why all the describing words match amiga.

In this context, solidaria means something like:

• supportive
• caring
• helpful
• showing solidarity with others

It does not usually sound like the English word solidary, which is rare in everyday English. A more natural English idea is that she is someone who supports and helps other people.

Por eso means that’s why, for that reason, or because of that.

It connects the first idea to the consequence:

• Mi amiga es modesta y solidaria; por eso todo el mundo se fía de ella.
• My friend is modest and supportive; that’s why everyone trusts her.

It is a very common linking phrase in Spanish.

Todo el mundo literally means all the world, but in everyday Spanish it means everyone.

It is extremely common and very natural. For example:

• Todo el mundo lo sabe = Everyone knows it
• Todo el mundo se fía de ella = Everyone trusts her

You could also say todos, but todo el mundo is often more idiomatic when talking about people in general.

Because todo el mundo is grammatically treated as singular in Spanish.

So you say:

• Todo el mundo se fía
• not Todo el mundo se fían

Even though the meaning refers to many people, the grammatical subject is singular.

Fiarse de means to trust or to rely on someone or something.

Important points:

• It is a pronominal verb: fiarse
• It normally takes de

Examples:

• Me fío de ti = I trust you
• No me fío de eso = I don’t trust that
• Se fía de ella = He/She trusts her

So in your sentence, todo el mundo se fía de ella means everyone trusts her.

Because the verb is fiarse, not just fiar.

This is one of those verbs that are commonly used with a reflexive/pronominal pronoun:

• me fío
• te fías
• se fía
• nos fiamos
• os fiáis
• se fían

Without se, you would usually be dealing with a different meaning of fiar, such as to sell on credit in some contexts.

So se fía de ella is the correct structure for trusts her.

Because fiarse is followed by de + person/thing.

So Spanish needs the preposition de, and after a preposition you use a prepositional pronoun:

• de mí
• de ti
• de él
• de ella
• de nosotros/as

That is why you say:

• Me fío de ella
• not Me la fío
• not Me le fío

The structure is fixed: fiarse de alguien/algo.

Because once mi amiga has already been mentioned, Spanish naturally uses the pronoun ella to avoid repetition.

So:

• Mi amiga es modesta y solidaria; por eso todo el mundo se fía de ella.

sounds much more natural than repeating the full noun phrase again:

• ...por eso todo el mundo se fía de mi amiga.

Both are grammatical, but de ella is smoother because the referent is already clear.

The accent mark shows the correct stress and helps indicate that í and a are pronounced in separate syllables.

So fía is pronounced roughly as FEE-ah, not as one merged syllable.

This happens because the stressed í forms a hiatus with the following vowel.

You see the same pattern in forms like:

• confía
• envía
• sonríe (similar idea, though with a different vowel combination)

They are very close, and both can often be translated as to trust, but they are not always used in exactly the same way.

Common patterns:

• fiarse de alguien/algo
• confiar en alguien/algo

Examples:

• Me fío de ella
• Confío en ella

Both can mean I trust her.

A useful learner point:

• fiarse de often emphasizes personal trust or whether you feel someone/something is reliable
• confiar en can also mean to have confidence in

In this sentence, se fía de ella is very natural.

The semicolon links two closely related ideas:

1. Mi amiga es modesta y solidaria
2. por eso todo el mundo se fía de ella

It works a bit like a stronger pause than a comma. In normal writing, you could also often see a full stop:

• Mi amiga es modesta y solidaria. Por eso todo el mundo se fía de ella.

A comma is sometimes used informally, but the semicolon makes the connection clear while keeping both parts in one sentence.

Yes. That would also be correct and natural.

Compared with the original:

• por eso = that’s why / for that reason
• así que = so
• se fía de ella = trusts her
• confía en ella = trusts her / has confidence in her

So your version is a good alternative. The original sentence is just using a different but very common set of structures.