How can a new cellular operator make iPhone preset the new operator's APN ?

It depends on the carrier. For the major carriers, there is a standard activation policy that automatically sets the APN.

For MVNOs who buy at wholesale rates from the the major carriers and sell to retail users it can be different. In many cases the APN set by the activation policy is not correct for these carriers and has to be set manually by the user. In such cases Apple is not setting the APN the user needs.

From what I know, they will have to provide a carrier file to Apple for distribution. That is a question the network would have to ask Apple. That really isn't something for a user to user support forum.

Apple and its carrier partners agree on specific carrier profiles that will be pushed to the phone when cellular service is provisioned on the device. Details of these arrangements are not in the public domain.