
Explanation
Which settings are swapped?
When you clone configuration from another deployment slot, the cloned configuration is editable. Some configuration elements follow the content across a swap (not slot specific), whereas other configuration elements stay in the same slot after a swap (slot specific). The following lists show the settings that change when you swap slots.
Box 1 : On
Settings that are swapped:
General settings, such as framework version, 32/64-bit, web sockets
App settings (can be configured to stick to a slot)
Connection strings (can be configured to stick to a slot)
Handler mappings
Public certificates
WebJobs content
Hybrid connections *
Virtual network integration *
Service endpoints *
Azure Content Delivery Network *
Features marked with an asterisk (*) are planned to be unswapped.
So web sockets settings will be swapped. So Production will have web sockets settings from "Off" to " On" after the swap slot.
Box 2: App1-prod.contoso.com
Settings that aren't swapped:
Publishing endpoints
Custom domain names
Non-public certificates and TLS/SSL settings
Scale settings
WebJobs schedulers
IP restrictions
Always On
Diagnostic settings
Cross-origin resource sharing (CORS)
So Custom domain names will not be swapped. So Production will have Custom domain names of its own after the swap slot.

Reference:
https://docs.microsoft.com/en-us/azure/app-service/deploy-staging-slots#what-happens-during-a-swap