Phụ tùng ở trong tiệm sửa xe.

It can be either. Vietnamese nouns usually do not change form for singular and plural, so phụ tùng can mean:

• a spare part
• spare parts
• parts in general

The exact meaning depends on context.

In a sentence about location, ở works like to be located at/in. It is doing the job that English often expresses with is/are plus a preposition.

So:

• Phụ tùng ở... = The spare parts are located...

Vietnamese does not need a separate word exactly like English are in this kind of sentence.

They are related, but not identical.

• ở marks location: be at / be in
• trong means inside

So ở trong means are inside.

Compare:

• Phụ tùng ở tiệm sửa xe. = The spare parts are at the repair shop.
• Phụ tùng ở trong tiệm sửa xe. = The spare parts are inside the repair shop.

Adding trong makes the location more specific.

It is a compact Vietnamese noun phrase:

• tiệm = shop
• sửa = repair / fix
• xe = vehicle

So tiệm sửa xe literally means something like shop repair vehicle, which English naturally turns into vehicle repair shop or repair garage.

Not necessarily. Xe is a general word for vehicle. It can refer to:

• a motorbike
• a bicycle
• a car
• another kind of vehicle

In everyday Vietnamese, tiệm sửa xe often suggests a motorbike repair shop unless the context makes it clear that cars are meant.

A little. The sentence structure is basically:

• Phụ tùng = the thing being talked about
• ở trong tiệm sửa xe = where it is

So the pattern is:

• [thing] + [location]

Also, inside tiệm sửa xe, the words that describe the shop come after tiệm. Vietnamese often puts modifiers after the main noun.

Usually not if you want a full sentence.

• Phụ tùng ở trong tiệm sửa xe. = a complete sentence
• Phụ tùng trong tiệm sửa xe = usually sounds like a noun phrase, such as the spare parts in the repair shop...

So ở is normally needed to make it a complete location statement.

Yes, but the nuance changes.

• tiệm sửa xe = repair shop; very normal
• gara = garage, especially for cars
• cửa hàng = store/shop, but less specifically a repair place
• chỗ sửa xe = place where vehicles are repaired; more informal

So tiệm sửa xe is a natural and common choice in this sentence.