Saapumisaulassa odotan ystävääni, joka tulee toisella lennolla.

Finnish very often makes compound words, where English would use several separate words.

saapumisaulassa breaks down like this:

• saapumis- = arrival
• aula = hall / lobby
• -ssa = in

So the core noun is saapumisaula = arrival hall, and saapumisaulassa means in the arrival hall.

The ending -ssa is the inessive case, which usually expresses being inside something.

This is about the object case.

The verb odottaa takes its object in the partitive case, so:

• ystävä = friend
• ystävää = friend (partitive)
• ystävääni = my friend (partitive + possessive suffix)

So after odottaa, Finnish normally uses the partitive:

• odotan bussia = I’m waiting for the bus
• odotan ystävääni = I’m waiting for my friend

Using ystäväni here would sound wrong, because odottaa does not normally take that form of the object.

The ending -ni is the 1st person possessive suffix, meaning my.

So:

• ystävä = friend
• ystäväni = my friend
• ystävääni = my friend, in the partitive form

Finnish often marks possession directly on the noun with a suffix. In many situations, that suffix can do the job that my does in English.

Because Finnish can express possession with a possessive suffix alone.

So instead of saying:

• minun ystävääni

the sentence simply uses:

• ystävääni

That is very natural Finnish.

You can sometimes see both a pronoun and a possessive suffix together, especially for emphasis or clarity:

• minun ystäväni

But in an ordinary sentence like this, the suffix -ni is enough.

joka is a relative pronoun, equivalent to English who, which, or that.

In this sentence, it refers back to ystävääni:

• ystävääni, joka tulee... = my friend, who is coming...

So the part after joka gives extra information about the friend.

A useful way to think of it:

• joka starts a relative clause
• it links that clause to the noun before it

In Finnish, a relative clause introduced by joka is normally separated with a comma.

So:

• ystävääni, joka tulee toisella lennolla

This is standard punctuation in Finnish. In English, comma use depends more on whether the clause is restrictive or non-restrictive, but Finnish is more regular here: the relative clause is set off with a comma.

Finnish does not have a separate future tense like English does.

The form tulee is present tense, but present tense in Finnish often covers both:

• present time
• future time, if the context makes it clear

So joka tulee toisella lennolla can naturally refer to someone who is coming / will come on another flight.

Context tells you whether it is present or future.

Both words are in the adessive case, marked here by -lla:

• toinen → toisella
• lento → lennolla

The adessive often means on, at, or by, but with travel expressions it can also be used in idiomatic ways.

Here toisella lennolla means something like:

• on another flight
• by another flight

It is just the natural Finnish way to express that the person is arriving on a different flight.

It can mean either, depending on context.

The basic word toinen can mean:

• second
• another / other

In this sentence, it is most naturally understood as another or a different flight.

So the idea is not necessarily that it is literally the second flight, but rather a different flight from the one being referred to or expected.

Yes, Finnish word order is fairly flexible.

This sentence begins with Saapumisaulassa to set the scene first:

• In the arrival hall, I’m waiting for my friend...

That sounds natural because Finnish often places known/background information early in the sentence.

Other orders are possible, for example:

• Odotan ystävääni saapumisaulassa.

That is also grammatical, but the emphasis shifts a little. Starting with Saapumisaulassa gives more prominence to the location.

Because Finnish verbs already show the person.

• odotan = I wait / I am waiting

The ending -n tells you it is first person singular, so minä is not necessary.

You could say:

• Minä odotan ystävääni...

but that usually adds emphasis. In ordinary Finnish, the pronoun is often omitted.

Because of both meaning and sentence structure.

The relative clause comes right after ystävääni, so the natural antecedent is ystävä:

• ystävääni, joka tulee...

Also, semantically, a friend can come on another flight, but an arrival hall cannot. So the reference is clear.

In Finnish, relative clauses usually follow the noun they describe, just like here.

Usually I am waiting fits the situation better in English, but Finnish uses the same simple present form:

• odotan

Finnish present tense often corresponds to either:

• English simple present, or
• English present continuous

So in context, odotan ystävääni is very naturally understood as I’m waiting for my friend.