Can I play a game offline or do I need internet access to play?

Sorry for a simple question: If I develop a game using UNITY, can I design the game to run allways “offline” without any Internet access, once it has been deployed, payed for and downloaded from a customer?
Does a customer need any access to a UNITY-server during installation?

The DRM managing is your own choice.
You can develop a game that never has to be connected to Internet ever - just a simple .exe file.