We use 'who' and 'whom' for people and 'which' for things. We can use 'that' for people or things.
We use relative pronouns:
• In defining clauses: 'after' a noun, to 'make it clear' which person or thing we are talking about.
E.g. i) The dress 'that' Amy designed
ii) The man 'who' invented the light bulb
• In non–defining clauses: to tell us more about a person or thing:
E.g. i) My mother, 'who' was born in Mumbai, has always been a great traveler.
ii) Leonardo DiCaprio, 'who' is 41, has just won an Oscar.
But we do not use 'that' as a 'subject' in non–defining relative clauses. 'That' can never be used in non–defining clauses.
'My aunt, that lived in London, gave me the red dress – is incorrect.