For gay parents, i want to underatand gay parenting / fmaily dynamics? How do you explain to your child zbout their missing biological parent ?

My 8 and 13 yr old are now learning some kids can have two moms or dads and asked me,how is that possible. Specially one a baby comes from mommy and daddy so what happens to the mommy or daddy? How can thos kids be born with mommy or daddy?