Ana e Júlia confiam uma na outra.

Confiam is the 3rd person plural present indicative form of confiar.

So:

• eu confio = I trust
• você/ela confia = you/she trusts
• eles/elas confiam = they trust

Because the subject is Ana e Júlia (two people), the verb has to be plural: confiam.

Because together they make a compound subject.

• Ana = one person
• Júlia = one person
• Ana e Júlia = two people

In Portuguese, just like in English, a subject joined by e normally takes a plural verb:

• Ana e Júlia confiam... = Ana and Júlia trust...
• Pedro e Marcos trabalham... = Pedro and Marcos work...

Literally, uma na outra means something like one in the other.

In Portuguese, this is a common way to express a reciprocal meaning with certain verbs. In English, we often just say each other, but Portuguese often builds the idea more explicitly.

So:

• Ana e Júlia confiam uma na outra = Ana and Júlia trust each other

The structure is:

• uma = one
• na outra = in the other

Together, it creates the reciprocal idea: one ... the other.

Yes — confiar normally takes em when it means to trust:

• confio em você = I trust you
• ela confia em Ana = she trusts Ana

In uma na outra, that em is still there, but it has combined with the feminine article a:

• em + a = na

So:

• confiar em a outra → confiar na outra

That is why the sentence has uma na outra and not a separate em.

Because both people are female: Ana and Júlia.

Portuguese marks gender in this expression:

• uma na outra = when both are feminine
• um no outro = when both are masculine, or in a mixed-gender pair

Examples:

• Ana e Júlia confiam uma na outra.
• Paulo e Bruno confiam um no outro.
• Ana e Paulo confiam um no outro.
  Masculine is the default in mixed groups.

Because the preposition depends on the verb.

With confiar meaning to trust, the pattern is confiar em alguém. So the reciprocal form uses em:

• uma na outra
• um no outro

You use uma à outra / um ao outro with verbs that take a, not em.

Compare:

• Ana e Júlia escrevem uma à outra.
  = They write to each other.
  (escrever a alguém)

• Ana e Júlia confiam uma na outra.
  = They trust each other.
  (confiar em alguém)

So the preposition changes with the verb.

Yes, you could.

• Ana e Júlia confiam uma na outra
• Ana e Júlia confiam entre si

Both can mean Ana and Júlia trust each other.

A useful difference:

• uma na outra sounds more direct and concrete for two people
• entre si can sound a bit more formal or less personal, and it is also very useful with larger groups

So for two people, uma na outra is extremely natural.

Usually, for the reciprocal meaning each other, it is better to keep the full expression:

• uma na outra

If you say only na outra, it may sound incomplete or may suggest a different context, as if one person trusts the other one rather than clearly expressing mutual trust.

So if your meaning is they trust each other, the safest natural form is:

• Ana e Júlia confiam uma na outra.

This exact singular pattern is most naturally used for two people:

• uma na outra
• um no outro

For larger groups, Portuguese often uses other forms, such as:

• uns nos outros
• umas nas outras
• entre si

Examples:

• Os amigos confiam uns nos outros. = The friends trust one another.
• As meninas confiam umas nas outras.
• Os colegas confiam entre si.

So yes, uma na outra strongly suggests a pair.

Here it is closer to one than to the indefinite article a/an.

In this structure, uma ... outra works like one ... the other:

• uma na outra = one in the other
• uma à outra = one to the other
• uma com a outra = one with the other

So although uma can often mean a, in this sentence it is really part of a reciprocal pattern meaning one ... the other / each other.