You can use your own Nuget Feed (or Nuget server how you called it) for Packages that have to be executed by the robots, but not for Activities.
If you have your own nuget feed, go to Settings in Orchestrator and navigate to the Deployment tab. If Orchestrator is version 18.3 or 18.4, make sure you turn off “Internal deployment”. Then you need to specify the complete URL of your nuget feed (including the protocol, http or https).
UiPath Studio will be able to publish there if it extracts the API key from Orchestrator.
For Activities, we the have the LIBRARIES menu item on the left. You can upload new activities or new versions of an activity with the button on the upper right. The robot will try to download from Orchestrator every activity pack that is listed in the Dependencies section of project.json.