Telling my daughter about her biological father

I have a 10 year old daughter, and a boyfriend of 9 years who has been her “daddy” since the day he met her, she had just turned 1. She started calling him daddy on her own as she grew up with him around and he had loved her as his own since day one. She did have somewhat of a relationship with her bio dad at first, until she was about 3 1/2. She never called her bio dad “daddy” and only saw him a couple times a year until he disappeared completely. Now she’s 10 and me and BF are having a new baby so she’s asking questions about when I was pregnant with her and when she was a baby and I don’t want to lie to her, I never wanted to lie to her about the truth we just never felt it was a good time to talk to her about all this complicated stuff but now I feel like I owe it to her and want to be as open as possible. Looking for any tips and advice on how to talk to her and not break her heart or make her feel weird in anyway. My BF is her dad in every way and will be forever, willing to legally adopt her and everything and her bio dad hasn’t popped up since she was 4! I’m nervous to have this talk with her.