I was honest, and said, to help people stop having babies if tey don't want them.
At first this was enough for dd, later she asked how they worked, and I told her.
What they want to know, they will ask. Tell them what they want to know and wait to see if they want to know more.
Just because, for instance, they ask about a condom, it doesn't mean they need to know everything about sex and reproduction. they might, but again, they might not.
Over the years dd has asked, and I have answered question. We have now just about covered it all, without resorting to the 'let me sit you down and have a chat' routine, which I remember as toe curlingly awful with my mother.
We run a very open house (except the loo door ). They kids know if they ask us antything, we answer them honestly.