If a Parent was originally a resident in Nevada as a minor; has court ordered Child support visitation on a child and physical custody... and has moved to SC ( since becoming an adult) with child and needs to modify Court order which court would have Jurisdiction to do the modification?

Which ever court made the original order.

It's rather unclear from your post, so further information would be helpful.

It depends on where your co-parent resides. If your custody and support orders originated in Nevada and the child's other parent still resides in Nevada, Nevada has jurisdiction and you'd have to file for modification there.

If the other parent has moved to a different state then you might be able to modify child custody in SC but support would have to be modified in the state in which the other parent resides.

We need more facts to give a specific answer.

The answer to your question is found in two uniform acts:

As to child custody: see the Uniform Child Custody Jurisdiction and Enforcement Act (UCCJEA).

As to child support: see the Uniform Interstate Family Support Act (UIFSA).