Learn Russian: В городе много молодёжи. - There are many young people in the city.

The base word for "young people" or "youth" is молодёжь. After words expressing quantity like много (a lot / many), Russian always uses the Genitive case. Because молодёжь is a feminine noun ending in a soft sign (-ь), its Genitive ending is -и, making it молодёжи.

"Young people" is plural in English. Why does молодёжь act like a singular noun?

In Russian, молодёжь is a collective noun. It refers to a group of people but is grammatically singular and feminine, much like the English word "youth". Because it is singular, you do not use a plural Genitive ending here.

Why is there no verb like "are" in this sentence?

In Russian, the present tense of the verb "to be" (is/are) is almost always left out. Literally, this sentence is "In city a lot of youth." To say there were many young people in the past, you would add the neuter past-tense verb было (because it pairs with the quantity word много): В городе было много молодёжи.

Why is it В городе with an -е at the end instead of just В город?

The sentence tells us a location (where the young people are), not a direction of movement (where someone is going). To show a static location with the preposition в (in), the noun город (city) must be in the Prepositional case, which usually adds an -е ending.