La zomo restas en la sako apud la kamerao.

Because they are nouns. In Esperanto, a basic noun ends in -o.

• zomo = a noun
• sako = a noun
• kamerao = a noun

The verb restas ends in -as, which marks the present tense. So this sentence is also a nice example of Esperanto’s regular endings:

• -o = noun
• -as = present-tense verb

restas comes from resti, meaning to remain or to stay.

So:

• La zomo estas en la sako = the zoom is in the bag
• La zomo restas en la sako = the zoom stays/remains in the bag

Using restas adds the idea that it continues to be there, or that it is being left there.

Each definite noun phrase normally has its own la.

So here you have:

• la zomo = the zoom
• la sako = the bag
• la kamerao = the camera

In English, learners sometimes expect one the to somehow carry across more than one noun, but in Esperanto it is very normal to repeat la with each separate noun phrase.

Because none of them is a direct object here, and the sentence is describing location, not movement toward somewhere.

• en la sako = in the bag
• apud la kamerao = next to the camera

Those are prepositional phrases showing where something is.

A very useful contrast is:

• en la sako = in the bag
• en la sakon = into the bag

So if there were movement, you might see -n after en, for example:

• Mi metas la zomon en la sakon = I put the zoom into the bag

But in your sentence, the zoom is simply staying in one place, so no -n is needed.

apud means beside, next to, or by.

It expresses nearby position, usually fairly close. In this sentence, it tells you that the bag is next to the camera, or at least that next to the camera is the relevant nearby location.

It is more specific than some other location words. For example:

• apud = next to / beside
• ĉe = at, with, at the place of

So apud la kamerao is specifically a next to the camera kind of idea.

Most naturally, it describes the bag:

• en la sako apud la kamerao = in the bag that is next to the camera

That is the most likely reading because apud la kamerao comes right after la sako.

If you wanted to make that completely explicit, you could say:

• La zomo restas en la sako, kiu estas apud la kamerao.

That clearly means The zoom remains in the bag, which is next to the camera.

Yes. Esperanto word order is fairly flexible, because endings and prepositions do a lot of the grammatical work.

Still, La zomo restas en la sako apud la kamerao is the most neutral and natural order here.

You can reorder parts for emphasis, but doing so can make the sentence less clear. For example, moving apud la kamerao around might make a learner wonder whether it describes the bag or the whole situation.

So the given order is a good standard choice.

Only if you want a different meaning.

With la, the nouns are definite/specific:

• la zomo = the zoom
• la sako = the bag
• la kamerao = the camera

Without la, the sentence would sound more indefinite or less specific, something closer to:

• a zoom remains in a bag next to a camera

Whether that works depends on the context, but it is not the same meaning.

Yes. Esperanto can use borrowed roots, but they still follow Esperanto grammar.

So even if a word comes from another language, once it is in Esperanto it takes normal endings:

• kamerao = noun, so it ends in -o

That is one of the most important habits to get used to: even internationally familiar words become fully regular Esperanto words.