Really this should have been discussed in an age appropriate way from day one.
My son is adopted but young enough not to have conscious memories of me not being his Dad. From very early on I spoke about his birth parents, I have some pictures of his birth mum (but sadly know nothing about his birth dad) that are in his life story book, he has others in a draw in his bedroom that he sometimes looks at.
In an age appropriate way (he’s five in June) he knows why his birth mum couldn’t look after him, he knows that she hurt him and he will sometimes point to a scar and say how it happened. It’s really important that he is told the truth, people who keep you safe tell the truth. An eight year old is more than able to know in an age appropriate way if a birth parent was abusive etc.
A big thing is the better the understanding of sex the easier it is, my son has known how babies are made for about a year, this has made it easier for him to understand that he has a birth mum and a birth dad.
You have to remember child cope with things very differently compared to adults, my son regularly pops up with a question about his birth parents, usually at the most awkward time possible, he will also tell other people little snippets of information. A couple of weeks ago he got out a picture of his birth mum and explained (fairly accurately) who she was to my boyfriend. Children need to know they can say/ask things when it suits them, even if it is a bit awkward for us.
There are lots of good resources online for life story work. We have a book I have made, sometimes I read it with my son, sometimes he looks on his own, sometimes he shows other people, I keep it upto date so the main focus is on us rather than the main focus being on his birth parents.