On laptops with Nvidia Optimus, the Intel integrated GPU always drives the screen. The Nvidia GPU acts as a co-processor. When a game needs a frame rendered, it's rendered on the Nvidia GPU. The video drivers then pass the frame from the Nvidia GPU to the Intel GPU for display.
With a few exceptions, there is no way to hook up the Nvidia GPU directly to the screen. A few laptops are set up with the Intel GPU driving the screen, the Nvidia GPU driving the external video port. And a handful of laptops can in fact have the Nvidia GPU drive the screen (there's a BIOS setting). But these are very rare, and if your laptop isn't one of them there's no way to do it via software. The Nvidia GPU physically needs to be connected to the video output.
That said, Nvidia Optimus works well most of the time. If you open up Nvidia Control Panel (right-click the desktop), you can select which apps use the Nvidia GPU, which use the Intel GPU. You can even set the default to using the Nvidia GPU.
There are a handful of older games where Optimus doesn't work. These games were coded back in the days when computers only ever had one GPU. So the game searches for which GPU is installed, finds the Intel GPU (since it's driving the screen), and stops searching. It never looks for the Nvidia GPU so it never finds it, and as a result can only use the Intel GPU.
If you're using an OS without Intel + Nvidia drivers for Optimus, then you are SOL. Without proper drivers, only the Intel GPU can be used.