How to connect external linux installed cpu with no monitor as input to windows installed lenovo T6 laptop and make the laptop monitor as cpu monitor?

Sorry, not possible.

Thank you for your answer but can I connect it through the sabrent usb 2.0 vga or vdi adapter( that I saw in you tube video). Is it possible?

I don't know this piece of hardware so cannot comment on it.

Bear in mind that laptop screens are designed purely for the connected CPU.
They can output the display to an external device, but as they have no VGA input they cannot display another directly attached machine.

If you had an external monitor on a desktop, you could use a simple KVM switch to share the monitor, keyboard and mouse among multiple PCs. But this will NOT work with a laptop, since a laptop has no output for the monitor, keyboard or mouse.