Use a/an for jobs; usually no article with plural nationalities; special cases for groups.
Job: a/an + singular job noun. Nationality adjective: no article. National group: the + plural (often).
Do not use the + job for general identity unless you mean a specific person (the doctor we saw).
Is it a job (countable noun) or nationality adjective?
💡 Nationality as adjective: She is Italian (no article). As a noun: She is an Italian (less common, often sounds formal).
✓She is a doctor.
✗She is doctor.
Jobs are singular countable nouns and usually need a/an.
✓He is Brazilian.
✗He is a Brazilian (when you mean adjective identity)
Nationality adjectives generally take no article.
✓The French are famous for their food.
We use 'the' with some nationality groups to mean the people.