It's "outskirts are" . Since "outskirts" is a noun that only has a plural form, it takes a verb in the plural.

It's "outskirts are" (not "outskirts is"). Since "outskirts" is a noun that only has a plural form, it takes a verb in the plural.

The outskirts are the best place to live in Madrid.

The outskirts is the best place to live in Madrid.

Follow the same convention with other verbs to say, for example, "outskirts have" (not "outskirts has").

The outskirts of Belfast have maintained their rural feel.

The outskirts of Belfast has maintained their rural feel.

Plural nouns, like "outskirts", take a determiner or pronoun in the plural.

The city's outskirts are interesting. They are being revived by innovative manufacturing activities. (not "It is being revived...")

Because the outside part of a city or town has no borders, we say "on the outskirts", not "in the outskirts".

They live on the outskirts of the city.

Be also aware that you cannot use the article "a" with plural nouns.

The outskirts of the city are near his home.

Follow the same strategy with other plural nouns, such as "suburbs" or "surroundings".