Learn Italian: Si è autonomi se si fa quello che si ama. - One is autonomous if one does what one loves.

This is a special rule for the impersonal si. When you use si with the verb essere (to be) to say 'one is' or 'people are', the verb stays in the singular form (è), but the adjective describing the subject must be in the plural form. That is why we use the plural autonomi instead of the singular autonomo.

Does autonomi have to be masculine plural, or could it be feminine?

Because this sentence is a general abstract statement applying to anyone, we use the default masculine plural autonomi. However, if context made it clear that 'one' referred exclusively to a group of women, you would use the feminine plural and say Si è autonome.

Why is 'what' translated as quello che here instead of just cosa?

When 'what' does not ask a question but instead means 'that which' or 'the thing that' in the middle of a sentence, Italian uses quello che (or sometimes ciò che). You would use cosa for actual questions, like asking someone what they are doing.

This sentence uses si three times (si è, si fa, si ama). Do they all serve the exact same purpose?