Learn Hardcore Russian: Люди, которые ходят в парк вечером, любят тишину. - People who go to the park in the evening love the silence.

The noun is люди (people) – masculine plural in meaning, grammatically just plural.
So the relative pronoun also has to be plural → которые.

Compare:

Человек, который ходит в парк вечером, любит тишину. – One person, который (singular).
Люди, которые ходят в парк вечером, любят тишину. – People, которые (plural).

You would not use кто here, because кто is a question/indefinite pronoun (who?), not a relative pronoun that introduces a clause describing a specific noun.

Why are there commas around которые ходят в парк вечером? Are they necessary?

Yes, both commas are necessary.

Которые ходят в парк вечером is a non‑defining (non‑restrictive) relative clause that gives extra information about люди:

Люди, которые ходят в парк вечером, любят тишину.
→ We are talking about people in general and then adding extra info: specifically, those who go to the park in the evening.

In Russian, non-defining relative clauses are always separated by commas on both sides.

If you removed the clause, the sentence would still be grammatically complete:

Люди любят тишину.

With a defining (restrictive) clause (rare with люди here), you might sometimes see different punctuation, but with describing like this, the commas are standard and expected.