How are SIAMESE or CONJOINED TWINS formed?

"So twins are formed by either the fertilized egg dividing into two babies or two eggs fertilized separately by the sperms. So where the fertilized egg is divided, they're going separated in that two identical twins formed. In conjoined twins they don't separate completely. There is a union between the twins partially. The union can be at the chest, can be at the abdomen, or it can be at the pelvis. Depending on the type of union we give different names for this conjoined twins. Many of conjoin twins do not actually survive till delivery. But few are born but they are conjoined they have complications sometimes they can even share their internal or. So based on how much of internal organs are shared. They can undergo surgery, get them separated and there are few successful surgeries perform on conjoined twins"