I wanted to use my old pc with hdmi port as a 2nd screen for my pc desktop that works
You can only partially use your laptop as a monitor. Actually, the idea is clever: working on two screens, but the second screen is the laptop. We'll show you how it works.
Yes, but I can extend the screen or just duplicate
Just try it, I've never done that before.
Only with additional hardware.
But there are modules with which this is possible. Can also order eBay, have to see what the name of the panel.
These are controllers, because you connect the panel removed from the laptop.
The problem is, most laptops have only HDMI outputs and no input.
Should there be a suitable adapter for your laptop, you can also remove the screen & use it like this.
But if a) consuming, b) is then quite a hobby design and c) not necessarily much cheaper than the same to buy a real monitor.