Ventilasjonsteknikeren undersøker ventilasjonsaggregatet.

Because Norwegian very often makes compound nouns by joining words together.

• ventilasjonstekniker = ventilasjon
  • tekniker
• ventilasjonsaggregat = ventilasjon
  • aggregat

This is much more common in Norwegian than in English. Where English might write ventilation technician as two words, Norwegian usually writes one compound word.

A useful rule:

• In Norwegian, if English has noun + noun, Norwegian often combines them into one word.

Those endings mark the definite form: the ...

Norwegian often adds the equivalent of the to the end of the noun.

• ventilasjonstekniker = ventilation technician

• ventilasjonsteknikeren = the ventilation technician

• ventilasjonsaggregat = ventilation unit / ventilation system unit
• ventilasjonsaggregatet = the ventilation unit

The different endings come from the noun’s gender:

• -en is a common definite ending for common gender nouns
• -et is a common definite ending for neuter nouns

So:

• en ventilasjonstekniker → ventilasjonsteknikeren
• et ventilasjonsaggregat → ventilasjonsaggregatet

Because in Norwegian, the definite article is usually attached to the noun as a suffix.

So instead of writing a separate word like English the technician, Norwegian often says:

• teknikeren = the technician
• aggregatet = the unit

That is completely normal Norwegian grammar.

You can sometimes see a separate determiner together with a definite noun, but that happens in special situations, especially with adjectives:

• teknikeren = the technician
• den dyktige teknikeren = the skilled technician

In your sentence, there is no adjective, so the suffix alone is enough.

The dictionary/basic forms are the indefinite singular forms:

• en ventilasjonstekniker
• et ventilasjonsaggregat

In the sentence, both nouns are in the definite singular:

• ventilasjonsteknikeren
• ventilasjonsaggregatet

So if you were looking these up, you would normally search for:

• ventilasjonstekniker
• ventilasjonsaggregat
• undersøke (the infinitive of the verb)

Undersøker is the present tense of undersøke = to examine / inspect / investigate.

So:

• å undersøke = to examine
• undersøker = examines / is examining

In Norwegian, the present tense does not change depending on the subject:

• jeg undersøker = I examine / am examining
• du undersøker = you examine
• han undersøker = he examines
• de undersøker = they examine

That is much simpler than English, because you do not have forms like I examine but he examines. Norwegian just uses undersøker for all persons.

Yes, this sentence has the normal subject–verb–object order:

• Ventilasjonsteknikeren = subject
• undersøker = verb
• ventilasjonsaggregatet = object

So it works like:

• The ventilation technician
  • examines
    • the ventilation unit

This is a straightforward main clause, so the order is very similar to English.

That -s- is a linking element used in some Norwegian compounds.

Compare:

• ventilasjon
  • aggregat
• becomes ventilasjonsaggregat

This -s- does not mean possession here. It is not the same as English ’s.

It is just a common way of connecting parts of a compound noun in Norwegian. Some compounds have it, some do not, and learners often just have to get used to which form is standard.

So:

• ventilasjonsaggregat is a normal compound form
• the -s- is structural, not possessive

You usually learn noun gender together with the noun itself.

For these words:

• en ventilasjonstekniker → common gender
• et ventilasjonsaggregat → neuter

That matters because gender affects the article and the definite ending:

• en ... → often definite -en
• et ... → often definite -et

A good habit is to memorize nouns with their article:

• en tekniker
• et aggregat

That will make later forms much easier.

Yes, grammatically you could.

• Teknikeren undersøker aggregatet.

That would mean The technician examines the unit.

But the longer compounds are more specific:

• ventilasjonsteknikeren = specifically a ventilation technician
• ventilasjonsaggregatet = specifically a ventilation unit / ventilation system unit

So the full sentence is more precise and technical.

It can cover several of those meanings depending on context.

Undersøke is a broad verb that can mean:

• examine
• inspect
• check
• investigate

In this technical sentence, examines or inspects is probably the most natural English idea. The exact nuance depends on the situation, but grammatically it is just an ordinary transitive verb taking a direct object.

A fully indefinite version would be:

• En ventilasjonstekniker undersøker et ventilasjonsaggregat.

That means:

• A ventilation technician examines a ventilation unit.

Compare the two versions:

• Ventilasjonsteknikeren undersøker ventilasjonsaggregatet.
  = The ventilation technician examines the ventilation unit.

• En ventilasjonstekniker undersøker et ventilasjonsaggregat.
  = A ventilation technician examines a ventilation unit.

This is a good pair to study, because it clearly shows how Norwegian marks definiteness.

The key sound here is ø, which does not exist in standard English.

Very roughly:

• ø sounds somewhat like the vowel in British English bird or burn, but with rounded lips

So undersøker is approximately:

• un-der-SUR-ker or un-der-SOE-ker

But that is only an approximation.

A few pronunciation notes:

• u in under- is not English oo
• ø is a rounded front vowel
• the stress is usually on sø: underSØker

The best way to learn it is by listening to native audio and repeating it.