Where are you from?
I'm from London.
But what does that mean. Let's look at me:
I was born in Cambridge.
I have lived in Cambridge, Bristol, Tokyo and London.
I've lived in London for 15 years - which is a most of my adult life.
Can I say "I'm from London" when I was born in Cambridge? Yes, is the answer. Imagine this conversation.
A: Where are you from?
B: I'm from Cambridge.
A: Oh, me too. Have you been to the new pub on the High Street?
B: No, I haven't been there for 15 years.
A: But you said you were from Cambridge?
B: I am, but I live in London.
Here the person said they were from Cambridge but didn't know anything about Cambridge. This is not helpful.
My rule is: say you are from the place you know the best.
Example conversations
A: Where are you from?
B: I'm from London.
A: Were you born there?
B: No, I'm originally from Cambridge.
Originally from = the first place you were from
A: Where are you from?
B: Cambridge originally, but I live in London.
A: Where are you from?
B: London, but I now live in Tokyo.
A: Were you born in London?
B: No, I was born in Cambridge but moved away 15 years ago.