My twins started asking the same question at around 4. We borrowed an age appropriate book from the library that explained things in simple terms to them. It seemed to satisfy their questions for now.

At that age I told my daughter that she already has eggs in her tummy and that these potentially can become babies at a later age and that this only can happen when a little seed is added to the egg. She was super excited to hear this and walked with dollies under her dress. I also told that once she is a teenager every month an egg will leave her body with a little blood.
I bought her an anatomic figure of a pregnant baby at that age and she found it most interesting.
It made it so much easier to answer her questions at a later age

my 5 year old always asks this question, we have had alot of pregnancy and new babies around us lately! ive at the moment just been telling her when your adult and you want a baby and thn magic happens and a baby starts growing in the my tummy, (writing that out makes it sound terrible) still trying to work out another way to tell her when i have my next.

I personally don’t think 5 is the right time for the birds and the bees talk. But I guess I would ask why she wants to know, what brought the question on etc.