Learn Turkish: Onlar genç arkadaşlar.

There is no separate word for 'are' in Turkish. Instead, the 'to be' verb is attached to the end of the sentence as a suffix. For the 'they' form, that suffix is -lar or -ler.

The lesson says -lar means 'they are', but isn't -lar just making arkadaş plural?

It is technically doing both! In Turkish, the regular plural suffix for nouns and the 'they are' suffix look exactly the same (-lar or -ler). Here, it makes arkadaş plural (friends), and the 'are' is naturally understood from the subject Onlar.

Why is it genç and not gençler? Shouldn't the adjective be plural too?

In Turkish, adjectives don't change to match the noun they describe. Even though arkadaşlar (friends) is plural, the adjective genç (young) stays in its base singular form. You should never say gençler arkadaşlar.

Can I drop Onlar and just say Genç arkadaşlar?

Yes, you can! Pronouns like Onlar are often dropped in Turkish because the context usually makes the subject clear. Just keep in mind that without Onlar, the phrase Genç arkadaşlar could mean 'They are young friends' or simply 'Young friends' depending on the conversation.