Chrome Remote Desktop allows you to view your computer from another computer or phone / tablet from anywhere online.
If you are at home, it's fast enough to stream video in real time.
In this example, I'm simulcasting what's on the main TV to an old tablet with a dodgy charging socket.
Another method for Windows only is Spacedesk - it gives you a virtual monitor over wifi, but no audio. Use my fm transmitter hack for that :)