Learn Russian: Мы встретили соседа, который живёт в этом шумном районе. - We met a neighbor who lives in this noisy neighborhood.

The word который takes its case from its job in its own part of the sentence. Here, 'who' is the subject doing the living, so it takes the Nominative case (который). The 'in' belongs to the neighborhood (в этом шумном районе), not the word 'who'. If the sentence was 'the neighborhood IN WHICH the neighbor lives', then you would use в котором.

Why does the word for neighbor end in -а (соседа)?

Because the neighbor is the direct object of the verb встретили (met), it must be in the Accusative case. For living beings (animate masculine nouns) like сосед, the Accusative case looks exactly like the Genitive case, meaning you add an -а to the end.

Why do the words этом and шумном have the -ом ending?

They are describing районе (neighborhood), which is in the Prepositional case because it follows the preposition в (in) to show location. The word район is masculine, and masculine modifiers in the Prepositional case usually take the -ом or -ем ending.

Is the comma before который required?

Yes! In Russian, you must always separate the main sentence from a 'who' or 'which' clause with a comma. Unlike English, where you can sometimes skip the comma (like in 'the neighbor who lives here'), Russian grammar strictly requires it every time.