Can I use my laptop screen and keyboard to control my desktop and use the desktop as the functional part and the laptop as just a screen and keyboard? could i just turn off all the internal parts in my laptop and use the desktop
You can connect to the desktop (depending on the relevant OSs) via RDC. Bring it up full screen on the laptop, and you're then 'using' the desktop resources. Just like you were sitting at a keyboard on the desktop.
USAFRet is spot on. You can access your desktop's desktop from your laptop via Remote Desktop or another similar application. You can't simply use your laptop's keyboard/display (directly) as you propose. Laptops just aren't designed to do that.