Learn Russian: Папарацци не любят, потому что они мешают жить. - People don't like paparazzi because they make life difficult.

This is an indefinite-personal clause. By leaving out the pronoun они (they), the sentence makes the subject generalized, meaning 'people in general'. If we added они, it would mean a specific group of people dislikes the paparazzi, rather than people as a whole.

If there is no subject in the first part, who does они refer to in the second part?

они refers to the папарацци. This is a great example of how Russian handles pronoun reference. In the first clause, the paparazzi are the direct object being disliked. In the second clause, they become the active subject because they are the ones doing the interfering (мешают).

Why doesn't the word папарацци change its ending, since it is the direct object here?

папарацци is a foreign loanword borrowed from Italian. Like many foreign loanwords in Russian (such as метро or кафе), it is indeclinable. It keeps the exact same form in all grammatical cases, whether it is the subject or the accusative direct object as it is here.

How does мешают жить translate to 'make life difficult'?

The verb мешать means 'to hinder', 'to interfere', or 'to be in the way'. When you pair it with the infinitive жить (to live), the literal meaning is 'they hinder living'. This is a common Russian expression for someone or something that constantly causes trouble, which we naturally translate as 'making life difficult'.