Learn Spanish (Latin America): En general, la gente es amable aquí.

In English, "people" is plural, so we say "people are". However, in Spanish, la gente is a singular feminine noun (think of it as a collective "crowd" or "group"). Because it is grammatically singular, it must take the third-person singular verb es.

Since this lesson is about uno, could I say uno es amable aquí instead?

You could, but it changes the meaning! Uno (one) is used for shared experiences that include the speaker. Saying uno es amable aquí means "one is friendly here" (implying that you are friendly here because that is the local norm). By using la gente, the speaker is observing the people around them as an outside group.

Why is the adjective amable singular here?

Adjectives must match the noun they describe in both gender and number. Because la gente is a singular noun, the adjective describing it must also be singular (amable, not amables). Even though it refers to multiple individuals, grammatically you are describing a single collective group.

Does en general work exactly like "in general" in English?