La ciudad recibe más de un millón de turistas en verano, por eso mi hermana prefiere visitarla en otoño.

Because the subject is la ciudad, which is singular: the city.
So the verb must also be singular:

• La ciudad recibe = The city receives
• Las ciudades reciben = The cities receive

Even though más de un millón de turistas is plural in meaning, it is the object of the verb, not the subject.

Yes, recibe literally means receives, from recibir. In Spanish, it is very natural to say that a city receives tourists, meaning it gets / attracts / has tourists.

So:

• La ciudad recibe turistas = The city receives tourists
• In more natural English, you might translate it as The city gets more than a million tourists or The city attracts more than a million tourists

Spanish often uses recibir in this kind of context.

They come from two different structures:

1. más de = more than
2. un millón de turistas = a million tourists

So the full phrase is:

• más de
  • un millón de turistas
• more than
  • a million tourists

Also, millón is a noun in Spanish, so it needs de before the thing being counted:

• un millón de habitantes
• dos millones de euros
• medio millón de personas

You cannot say un millón turistas.

Because millón is the number word being used, and the quantity is one million, so millón stays singular.

Compare:

• un millón de turistas = one million tourists
• dos millones de turistas = two million tourists

The noun after it, turistas, is plural because there are many tourists.

In Spanish, when talking about seasons in a general sense, it is very common to say:

• en verano = in summer
• en otoño = in autumn
• en invierno = in winter
• en primavera = in spring

This is similar to English, where you often say in summer rather than in the summer.

You may also hear en el verano or en el otoño, but without the article is very common, especially when speaking generally rather than about one specific summer or autumn.

Por eso means for that reason, so, or that’s why. It introduces a consequence.

In this sentence:

• La ciudad recibe más de un millón de turistas en verano, por eso mi hermana prefiere visitarla en otoño.
• The city gets more than a million tourists in summer, so my sister prefers to visit it in autumn.

By contrast, porque means because and introduces a cause.

Compare:

• Prefiere visitarla en otoño porque en verano hay muchos turistas.
  = She prefers to visit it in autumn because there are many tourists in summer.

So:

• porque = because
• por eso = that’s why / so / for that reason

Prefiere is the 3rd person singular present tense of preferir.

The subject is mi hermana, so the correct form is:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

This is a stem-changing verb: e → ie in most present-tense forms.

So:

• mi hermana prefiere = my sister prefers

Prefería would mean used to prefer / preferred in the imperfect, so it would change the meaning.

Because la is a direct object pronoun attached to the infinitive visitar.

• visitar = to visit
• la = her / it (feminine singular direct object)

So:

• visitarla = to visit it / to visit her

Here, la refers to la ciudad, which is feminine:

• la ciudad → visitarla

Spanish often attaches object pronouns to infinitives:

• quiero verlo = I want to see it / him
• vamos a comprarla = we are going to buy it
• prefiere visitarla = she prefers to visit it

Because la ciudad is the direct object of visitar, and visitar takes a direct object.

• visitar la ciudad = to visit the city
• Replace la ciudad with a pronoun → visitarla

For feminine singular direct objects, Spanish uses la.

• lo = masculine singular direct object
• la = feminine singular direct object
• le is usually an indirect object pronoun

So here, la is correct because the city is the thing being visited directly.

Because Spanish normally uses the definite article with common nouns like ciudad when they are specific or already known:

• la ciudad = the city

But with a possessive like mi, Spanish usually does not use the article:

• mi hermana = my sister
• not la mi hermana in standard modern Spanish

So the patterns are:

• la ciudad
• el coche
• mi hermana
• tu casa
• nuestro amigo

Yes, that is possible, but en otoño and en verano sound very natural when speaking generally.

There is often a slight difference:

• en otoño / en verano = general, broad seasonal time
• en el otoño / en el verano = can sound a bit more specific or stylistically heavier, depending on context and region

For a learner, the version in your sentence is an excellent standard pattern:

• en verano
• en otoño

Because in Spanish from Spain, the normal word for the season autumn/fall is otoño.

• otoño = autumn / fall

A literal idea like fall in the sense of leaves falling is not how Spanish names the season. So for Spain Spanish, otoño is the standard word you should use.

Yes. The sentence follows a very typical and natural Spanish order:

• La ciudad recibe más de un millón de turistas en verano
• por eso mi hermana prefiere visitarla en otoño

A few useful things to notice:

• The cause is stated first: the city gets lots of tourists in summer.
• Then por eso introduces the result: my sister prefers to visit in autumn.
• The pronoun is attached to the infinitive: visitarla.
• Time expressions like en verano and en otoño come naturally after the main information.

So overall, this is a very normal and useful sentence pattern in Spanish.