#adoption

What is the best way to explain to my 4yo son that he is adopted and he has two moms?

As you know my son is adopted and we do yearly visits with his biological parents. Last year we did FaceTime because of COVID but this year we will do it in person with the precautions needed for everybody to be safe of course. He is now about to be 4 and we have always spoken about adoption openly in the house as we don’t keep it a secret. I asked him if he remembered ( let’s refer to bio dad as XYZ and bio mom as ABC ) and he said, “who is XYZ and ABC?” I don’t know what wording to use for him to understand. I have told him that he grew in ABC’s tummy like his brother grew in mine but that’s about it. I get stuck on how to continue the explanation.