Use iPhone as display for computer?
Assuming I understand your question correctly, you want to use a device such as an iPhone as an extra display for your desktop computer. If so, you will need to use 3rd party software to do this, such as Duet Display. 

I have used this app to set an iPad up as an external monitor for an iMac and it works well for some workflows. How well it works for you will depend on what you're trying to do and the hardware you're using.

In any case, I know that Duet Display works with both iPads and iPhones and works with both Mac and Windows. However, I can not share any personal experience in using with an iPhone or Windows.

Hope this helps.