The borrowed GPU doesn't work in my current PC but works in my very old one (the one with the i7-2600) while my 1080 ti doesn't work in both.So a borrowed GPU worked - correct?
Try your GPU in another known working computer to determine if the GPU works.
Also test using different video ports and video cables.
Be methodical about testing different combinations to determine what (if anything) works and what does not work.
Change only one thing at a time.