Learn French: C'est le quartier où ils vont développer les transports en commun. - It is the neighborhood where they are going to develop public transport.

We use c'est because it is followed by a noun phrase with a determiner (le quartier). You generally use c'est before a modified noun, whereas il est is used before an adjective on its own.

Why is the word où used here instead of que?

In French, où is used as a relative pronoun for places and times. We need où here because it means 'where' or 'in which' the action takes place (in the neighborhood). You cannot use que here because que replaces a direct object, not a location.

Why is 'public transport' translated as les transports en commun rather than using the French word for 'public'?

Les transports en commun (literally 'transport in common') is the standard, idiomatic way to say 'public transportation' in French. While the word public exists, using it for transit sounds unnatural to native speakers.

Why does the sentence use vont développer instead of the simple future tense?

Ils vont développer is the futur proche (near future), which translates to 'they are going to develop.' Built with the present tense of aller plus an infinitive, it is the most common way to talk about planned or upcoming actions in everyday French.