Learn Italian: Coloro che criticano il consumismo preferiscono comprare poco. - Those who criticize consumerism prefer to buy little.

Both translate to "those who," but coloro che is more formal and elevated. You will see it often in literature, journalism, or speeches. In everyday spoken Italian, people almost always use quelli che.

Could I use chi instead of coloro che to mean "those who" here?

Yes, but you would have to change the verbs to the singular. Chi means "he who" or "anyone who" and always takes a third-person singular verb, even when referring to a plural group of people. The sentence would become: Chi critica il consumismo preferisce comprare poco.

Why is there an il before consumismo when we just say "consumerism" in English?

Italian requires definite articles before abstract nouns and broad concepts when speaking about them in a general sense. While English drops the article for broad concepts like consumerism, love, or society, Italian keeps it.

Why don't we need a preposition like di or a between preferiscono and comprare?

The verb preferire connects directly to an infinitive verb without needing a linking preposition. You just use preferire + the infinitive.