Emphasizing information: It was John who called. What I need is coffee.
It + be + emphasized part + who/that + rest. OR What + clause + be + emphasized part
It wasn't X who/that... What I don't want is...
Was it X who...? What is it that you need?
💡 Who for people; that for things/time/place. What-clefts use singular 'is' even with plural.
✓It was the noise that woke me up.
✗It was the noise who woke me up.
Use 'that' for things, 'who' for people.
✓What I like is your honesty.
✗What I like are your honesty.
What-clefts use singular verb.
✓It's you who should apologize.
✗It's you that should apologize.
Who preferred for people (that also acceptable).