Ni kolektis milionon da moneroj.

Because -is marks the past tense in Esperanto.

• kolektas = collect / are collecting
• kolektis = collected
• kolektos = will collect

So Ni kolektis means we collected.

The -n ending marks the direct object.

In this sentence, the thing that was collected is the whole quantity expression: milionon da moneroj. Because that whole phrase is the object of kolektis, miliono takes -n:

• Ni kolektis milionon da moneroj. = We collected a million coins.

This is a very common pattern in Esperanto with quantities.

Esperanto uses da after words that express quantity, measure, or amount.

Examples:

• glaso da akvo = a glass of water
• kilogramo da rizo = a kilogram of rice
• miliono da moneroj = a million coins

So da links the quantity word miliono to the thing being counted, moneroj.

Because coins are countable items, and the phrase refers to many of them.

After da, countable nouns are normally plural:

• cento da libroj = a hundred books
• miliono da moneroj = a million coins

If the noun is a mass noun, it may stay singular:

• litro da akvo = a liter of water

So moneroj is plural because individual coins are being counted.

Because the accusative -n goes on the main quantity word, not normally on the noun after da.

So Esperanto says:

• Ni kolektis milionon da moneroj.

Not normally:

• Ni kolektis milionon da monerojn.

The object is understood as a million of coins, so milionon carries the object marking.

Yes. That is also correct.

• milionon da moneroj = a million coins
• unu milionon da moneroj = one million coins

Often unu is omitted when it is obvious or unnecessary, just like English often says a million instead of one million.

Yes. miliono is a regular noun.

That means it can take normal noun endings:

• miliono = a million
• milionoj = millions
• milionon = a million as a direct object
• milionojn = millions as a direct object

For example:

• Milionoj da homoj venis. = Millions of people came.
• Li gajnis milionon. = He won a million.

Ni is the Esperanto pronoun for we.

Esperanto subject pronouns are:

• mi = I
• vi = you
• li = he
• ŝi = she
• ĝi = it
• ni = we
• ili = they

So Ni kolektis... simply starts with the subject, we collected...

Yes, Esperanto word order is fairly flexible because grammar endings show the roles of words.

The neutral, most natural order here is:

• Ni kolektis milionon da moneroj.

But other orders are possible for emphasis, such as:

• Milionon da moneroj ni kolektis.

That puts extra focus on a million coins. Still, for learners, the standard subject–verb–object order is usually best.

Here is the structure:

• Ni = we
• kolekt-is = collect-past
• milion-on = million-object
• da = of, used after quantities
• moner-oj = coin-plural

So the grammar is:

subject + past-tense verb + quantity object + da + plural noun

That makes Ni kolektis milionon da moneroj a very typical Esperanto sentence.