How the vaccine based on "non replicating viral vector" is produced?

As I understand, this kind of vaccine is a virus that can infect the cell, start synthesis of proteins encoded by its genome, but cannot finish the replication cycle till the proper end. It can have extra genes in its genome, encoding proteins that cause the wanted immunity to develop. Some important vaccines against COVID-19 are based on this approach.

The question is, how such an "incomplete virus" could be produced in large amounts? It would be logical to assume that it was replicating during the manufacturing process, but then why does it cannot any longer? The only hint I found is the usage of the "helper virus" that infects the same cell, so the "incomplete virus" can borrow some parts from the healthy "coworker". However this raises another question: the helper virus obviously replicates also itself, and how to discard it later without inactivating the "non replicating" one? It would just cause infection without any use if not discarded.

I have done some search over web and article databases but was not able to find a short and clear explanation of this method. The main focus of the works I found is always somewhere else. I understand that exact technology may be secret, but it is likely at least remotely based on some research work anyway.

How is a "non replicating vector" replicated to produce enough of it, for making a vaccine out of it?