Once the parental rights of the biological parents have been terminated, does that mean the relationship of all biological relatives are also terminated?

The termination of parental rights is a specific state court ruling, and each state may have different terms, policies and procedures that are followed.
Next even each ruling can differer if there are various attached motions invovled.

With that said, some states , not many I understand, have specific grand parents rights, which I don't believe would be terminated, it merely ends any legal responsibility of that parent. ( to allow for the child to be adopted normally by a new step parent)

But of course remember that in latter years the parent who gave up or lost their rights, may again be part of the child's life if the child wishes to know about them or see them.