Аптека находится внизу.

Yes. Russian normally does not use a present-tense form of to be in simple sentences.

So:

• Аптека внизу. = The pharmacy is downstairs.

is completely normal.

But находится is not just a substitute for is. It means:

• is located
• is situated
• can be found

So:

• Аптека находится внизу. = The pharmacy is located downstairs.

This sounds a little more explicit or formal than just Аптека внизу.

Аптека means pharmacy or drugstore.

Depending on the country and context, English speakers might also think of chemist’s or chemist’s shop. In Russian, аптека is the normal word for the place where you get medicine.

Its stress is:

• апте́ка

Внизу means:

• downstairs
• below
• at the bottom
• down below

In this sentence, the most natural translation is usually downstairs.

So Аптека находится внизу means the pharmacy is on a lower level, below the current place, or further down.

This is a very common Russian distinction:

• вниз = downward / down in the sense of movement
• внизу = down below / downstairs in the sense of location

So:

• Идти вниз = to go down
• Быть внизу = to be downstairs

In your sentence, the pharmacy is not moving. Its location is being described. So Russian uses внизу.

It is one word: внизу.

In modern Russian, learners usually treat it as an adverb meaning down below / downstairs.

So in this sentence:

• внизу is not working like a normal preposition + noun phrase
• it functions as a location adverb

That is why you do not need to worry about a separate case ending here in the same way you would with something like в доме or на столе.

Yes, находится is the 3rd person singular form of находиться.

The ending -ся often marks a reflexive verb, but in many verbs it does not have a strongly literal reflexive meaning in English. Here, you should learn находиться as a whole verb meaning:

• to be located
• to be situated

So:

• аптека находится = the pharmacy is located

Its stress is:

• нахо́дится

Yes, absolutely.

Both are natural:

• Аптека внизу.
• Аптека находится внизу.

The version without находится is shorter and very common in everyday speech.

The version with находится can sound:

• a bit more formal
• a bit more explanatory
• slightly closer to is located

So the choice is about style, not basic meaning.

Аптека is in the nominative singular.

It is the subject of the sentence:

• Аптека = the pharmacy

That is why it has its dictionary form here.

Yes. Russian word order is flexible.

The neutral order here is:

• Аптека находится внизу.

But you could also say:

• Внизу находится аптека.

This version puts more focus on the location first, something like:

• Downstairs is the pharmacy.

So the basic meaning stays the same, but the emphasis changes.

Not always. It depends on context.

Внизу can mean:

• downstairs in a building
• below something
• at the bottom

Examples:

• Он внизу. = He’s downstairs.
• Подпись внизу страницы. = The signature is at the bottom of the page.

In your sentence, because we are talking about a pharmacy in a building or place, downstairs is the most natural interpretation.

A natural pronunciation is:

• Апте́ка нахо́дится внизу́.

Stress:

• апте́ка
• нахо́дится
• внизу́

A rough English-style approximation would be:

• ap-TYE-ka na-HO-dit-sa vnee-ZOO

But it is better to copy the Russian stress pattern directly than rely too much on English spelling.

They are similar, but not exactly identical.

• внизу = downstairs / below
• на нижнем этаже = on the lower floor

So:

• Аптека находится внизу. = The pharmacy is downstairs.
• Аптека находится на нижнем этаже. = The pharmacy is on the lower floor.

The second version is more specific.
The first is more natural in many everyday situations.

It is mostly neutral.

• Аптека внизу. sounds a bit more conversational and compact.
• Аптека находится внизу. sounds a bit more complete and slightly more formal or descriptive.

Neither one is strange. Both are good Russian.