I'm running windows 10 and recently purchased a Dell projector. I can project only powerpoint presentations, nothing else except the laptop desktop picture.
If your desktop is extended then you may only see the 2nd desktop. If you mirror the display, you will see the same thing on both displays. Extended is usually better because you can drag other windows to the 2nd display if needed and users won't see what you're doing on the main display.
Whatever is on the laptop screen should be view able through the projector, a projector is just like a monitor except the image is projected onto a wall for example.
If your desktop is extended then you may only see the 2nd desktop. If you mirror the display, you will see the same thing on both displays. Extended is usually better because you can drag other windows to the 2nd display if needed and users won't see what you're doing on the main display.