Learn Russian: Это спокойный город.

When it means "This is", Это acts as a fixed introductory word and stays in its neutral form. If you wanted to say "This city is calm", pointing specifically at the city, you would use the masculine word for "this" to match the noun: Этот город спокойный.

Where is the verb "is" in this sentence?

Russian almost never uses the verb "to be" in the present tense. To say "This is a calm city", you simply put the words together literally as "This a calm city".

How do we know the gender of город?

Nouns that end in a hard consonant, like the д in город, are almost always masculine. This is one of the most reliable rules for identifying noun genders in Russian.

Why does the adjective спокойный end in -ый?

Adjectives change their endings to match the noun they describe. Since город is a masculine noun, спокойный takes the standard masculine adjective ending -ый.