[Macboatmaster]

I have not watched the YouTube video
So I do not know if the folowing is included

1. Check if the line shown on your image from the mobile camera
is present in the BIOS or POST screen
If it is the likely cause is the screen, or its connecting ribbon cable

2. Check if you can if it present on an external monitor
IF IT IS NOT this tends to confirm the above.

 

[Macboatmaster]

Re this

the monitor is a built-in laptop, not a desktop PC monitor, so there is no connecting cables,

and my post

or its connecting ribbon cable

the cable I refer to is the ribbon cable that connects the laptop monitor to the system board

You need to carry out the tests suggested