A caixa de transporte está ao lado da caixa de areia.

Because caixa is a feminine singular noun, so the definite article is a.

• a caixa = the box
• o carro = the car for a masculine noun

In Portuguese, articles are used more often than in English, so a caixa sounds natural where English might sometimes just say the carrier or the box depending on context.

Portuguese often builds noun phrases with noun + de + noun.

So here:

• caixa de transporte = literally box of transport → more naturally transport box / carrier
• caixa de areia = literally box of sand → in context, litter box

This structure is very common in Portuguese. English often prefers a compound noun, but Portuguese frequently uses de.

Here, de links the main noun to another noun that describes its function or content.

It can express ideas like:

• type: sapato de couro = leather shoe
• purpose: óculos de sol = sunglasses
• content/material: copo de água = glass of water

So:

• caixa de transporte = a box for transport
• caixa de areia = a box with/for sand

You should not always translate de literally as of. Its real job is often just to connect nouns naturally.

Because the sentence is talking about location, and Portuguese normally uses estar for location.

• A caixa está ao lado da caixa de areia. = location
• É would usually be used for identity, definition, or classification

Compare:

• A caixa está na sala. = The box is in the room.
• A caixa é grande. = The box is big.
• Isto é uma caixa. = This is a box.

For physical position, estar is the normal choice.

Ao lado de is a fixed expression meaning next to / beside.

Literally, it is something like at the side of:

• a + o = ao
• lado is a masculine noun: o lado = the side

So:

• ao lado de = at the side of / next to

Even though caixa is feminine, the phrase uses ao because ao belongs to lado, which is masculine.

Because in Portuguese, de + a contracts to da.

So:

• de + a caixa → da caixa

These contractions are standard and usually required.

Other common ones:

• de + o = do
• em + a = na
• em + o = no
• a + o = ao

So ao lado da caixa is made up of:

• ao = a + o
• da = de + a

Because the gender of lado has nothing to do with the gender of caixa.

In ao lado da caixa de areia, the article in ao agrees with lado:

• o lado → masculine
• therefore a + o = ao

Then da caixa is separate:

• a caixa → feminine
• therefore de + a = da

So there are two different noun groups:

• ao lado → based on o lado
• da caixa de areia → based on a caixa

Yes. In Brazilian Portuguese, ao lado de and do lado de can both mean next to / beside.

Examples:

• A caixa está ao lado da caixa de areia.
• A caixa está do lado da caixa de areia.

Both are natural.
Ao lado de is a very standard textbook form, and do lado de is also very common in everyday speech.

Yes, to some extent.

For example, you could switch the two noun phrases if the situation still makes sense:

• A caixa de areia está ao lado da caixa de transporte.

That changes the topic/focus, even though the spatial relationship is basically the same.

You could also front the location for emphasis, though this is less neutral:

• Ao lado da caixa de areia está a caixa de transporte.

The original sentence is the most neutral and straightforward order:

• subject + estar + location

Usually, no—not in this sentence.

Portuguese tends to use definite articles more often than English, especially with concrete nouns in specific situations.

So:

• A caixa de transporte = a specific transport carrier
• a caixa de areia = a specific litter box

If you removed the articles, it would sound less natural or would change the meaning toward something more generic or label-like.

The full sentence sounds natural because both objects are understood as specific, identifiable things.

In Brazilian Portuguese, caixa is usually pronounced roughly like KAI-sha.

A simple approximation:

• cai sounds like kai
• xa sounds like sha

So:

• caixa ≈ KAI-sha

The letter x in Portuguese has several possible pronunciations, and in caixa it sounds like sh.

Because areia is normally an uncountable noun, like sand in English.

So Portuguese says:

• caixa de areia

not usually a plural form here.

Even when the real-world meaning is litter box, the phrase still uses singular areia because it refers to the material in a general sense, not to separate countable units.

It can work as both, but here it is mainly a descriptive noun phrase built in a very normal Portuguese way.

Portuguese often forms expressions like this:

• caixa de papelão
• mesa de jantar
• roupa de cama

So caixa de transporte is perfectly natural as a transport box / carrier, especially in contexts like pets, moving, or storage.

Even if English might prefer a special term like carrier, Portuguese is very comfortable using this kind of noun + de + noun structure.