Why is Tromsø Airport's runway 01 referred to as 36 in the sign before the runway?
The runway number is changed to 18/36 due to the change of magnetic variation. The image on Google Earth must be outdated. This is from my current (AIRAC 2004) Jeppesen airport chart of ENTC (Tromso, Norway):



The video you linked was uploaded in March 2020, were the runway number had already been changed. I do not know how old the Google Earth image is. According to this thread on a simulation forum, the runway number was changed with AIRAC 1910, which came into effect on 12 Sep 2019.

By the way, both the numbers on the runway and the sign refer the magnetic heading. For more details, see How are runways numbered?.