Я выбрала сумочку, у которой длинный ремешок, и лёгкую блузку из хлопка.

Выбрала is the perfective past tense of выбрать, so it presents the action as completed: she chose / picked out the items.

• я выбрала = I chose / I picked out
• я выбирала = I was choosing / I used to choose / I spent time choosing

In this sentence, the speaker is talking about the result of the choice, so выбрала is the natural form.

Also, -а in выбрала shows that the speaker is female. A male speaker would say я выбрал.

Because they are direct objects of the verb выбрала.

In Russian, a feminine noun ending in -а / -я usually changes to -у / -ю in the accusative singular when it is the object of a verb.

• сумочка → сумочку
• блузка → блузку

So:

• Я выбрала сумочку = I chose a handbag
• Я выбрала блузку = I chose a blouse

This is one of the most common case patterns in Russian.

Because the adjective must agree with the noun it describes.

Since блузку is:

• feminine
• singular
• accusative

the adjective must also be:

• feminine
• singular
• accusative

So:

• nominative: лёгкая блузка
• accusative: лёгкую блузку

This is standard adjective-noun agreement in Russian.

У которой means something like which has or more literally at which there is.

The phrase у которой длинный ремешок describes сумочку:

• сумочка, у которой длинный ремешок = a handbag that has a long strap

Russian often uses у + genitive to express possession:

• у сумочки длинный ремешок = the handbag has a long strap
  • literally: at the handbag there is a long strap

Then that possession structure is turned into a relative clause:

• сумочка, у которой длинный ремешок

So которой is from которая, but it changes because it follows у, which requires the genitive.

Because it refers back to сумочка, but it must also fit the grammar of the phrase у которой.

Here is what matters:

1. It refers to сумочка, so it must be feminine singular.
2. The preposition у requires the genitive case.

So:

• nominative feminine singular: которая
• genitive feminine singular: которой

That is why Russian says:

• сумочка, у которой...

not:

• сумочка, которая... in this structure

Because ремешок is the thing being said to exist or be possessed.

In the structure:

• у которой длинный ремешок

the meaning is basically:

• which has a long strap
• literally: at which there is a long strap

The possessed thing, ремешок, stays in the nominative:

• длинный ремешок

So:

• у сумочки длинный ремешок
• у которой длинный ремешок

This is a very common Russian possession pattern.

Russian can sometimes use с for with, but in this kind of descriptive relative clause, у которой... is often the more natural and idiomatic choice.

Compare:

• сумочка с длинным ремешком = a handbag with a long strap
• сумочка, у которой длинный ремешок = a handbag that has a long strap

Both are possible, but they are slightly different in style:

• с длинным ремешком is a compact noun phrase
• у которой длинный ремешок is a full relative clause

The sentence uses the relative clause version.

Сумочка is a diminutive form of сумка.

Very roughly:

• сумка = bag
• сумочка = small bag / handbag / purse, often with a more personal, feminine, or affectionate tone

Diminutives in Russian can show:

• small size
• affection
• informality
• a certain style or nuance

So сумочка often sounds more like handbag or little purse than the more neutral сумка.

Because у которой длинный ремешок is a relative clause describing сумочку.

Russian normally places a comma before subordinate clauses, including clauses introduced by forms of который.

So:

• Я выбрала сумочку, у которой длинный ремешок...

The comma marks the start of the descriptive clause у которой длинный ремешок.

Because the same verb applies to both objects.

The sentence means:

• I chose [a handbag that has a long strap] and [a light blouse made of cotton].

Russian, like English, often omits the repeated verb when one verb clearly governs two items joined by и:

• Я выбрала сумочку ... и лёгкую блузку ...

You could think of it as:

• I chose X and Y

rather than:

• I chose X and chose Y

Because из means from / out of / made of, and it requires the genitive case.

• хлопок = cotton
• genitive singular: хлопка

So:

• блузка из хлопка = a blouse made of cotton

This is a very common pattern:

• из дерева = made of wood
• из металла = made of metal
• из шёлка = made of silk

Yes, Russian word order is flexible, but the given order sounds natural and clear.

The sentence starts with:

• Я выбрала = topic/action
• then names the two items chosen

The relative clause comes right after сумочку, because it describes that noun:

• сумочку, у которой длинный ремешок

If you moved things around too much, the sentence could become less clear or less natural.

So while Russian does allow variation, this version is a good standard word order for everyday speech and writing.

It connects two direct-object noun phrases, not just two bare nouns.

The two objects are:

1. сумочку, у которой длинный ремешок
2. лёгкую блузку из хлопка

So the structure is:

• Я выбрала [object 1] и [object 2].

Each object has its own description attached to it. This is very common in Russian and in English.