Learn Portuguese (Portugal): Eles são cidadãos americanos.

Words ending in -ão can have three different plural endings: -ões, -ãos, or -ães. The most common ending is -ões, but cidadão is one of the important exceptions that takes -ãos. You just have to memorize that one citizen is um cidadão and multiple are cidadãos.

Why isn't americanos capitalized like it would be in English?

In Portuguese, adjectives and nouns used for nationalities, languages, and days of the week are always written with a lowercase letter.

Why do we use são instead of estão here?

São is the 'they' form of the verb ser, which is used for permanent facts. Citizenship and nationality are considered permanent traits in Portuguese, so we use ser rather than estar (which we use for temporary things).

Why does americanos come after cidadãos?

In Portuguese, descriptive adjectives usually come after the noun they are describing. Because cidadãos is masculine and plural, the adjective also takes the masculine plural form americanos to match.