To your first question, the answer is yes, it’s advisable to leave the light on as a luminaries environment keeps the pupils smaller limiting the amount of wavelengths and light that gets to the retina, serving asa protective front

Two, I don’t think the size of the screen affects the illumination of the room, keep the room lit and if you can’t do that please stay quite distant from the screen as is done in cinema halls

Three, yes, this applies to using your fon, tv, YouTube etc. If you love doing some of these activities in the dark do well to make use of the warm features on your gadgets and do get a lens from your Optometrist for additional protection