Check whether your laptop supports being used as a display input
Use a wireless display feature if both devices support it
On Windows, open Settings > System > Projecting to this PC
On the main PC, press Windows + K or Windows + P and select the laptop
On the laptop, allow the connection if prompted
Use a remote desktop or screen-sharing app if direct display input is not supported
Install the same app on both devices
Connect both devices to the same Wi-Fi network if required
Start the app on both devices and pair them
Use an HDMI capture card if you want a wired connection
Connect the main PC output to the capture card
Connect the capture card to the laptop via USB
Open the capture software on the laptop
Set display mode to Extend on the main PC
Adjust display arrangement in Display Settings
Set the laptop as the secondary monitor
Change resolution and scaling if needed
Disconnect by ending the wireless session, closing the app, or unplugging the capture device