A little help for a newbie

I have a Galaxy Tab S3 and I don't understand what should be a simple thing. When watching YouTube and I go full screen everything but the video goes away. Unfortunately the video is in what I can best describe as watching something on TV in the letterbox format. It is nice that all of the other stuff doesn't show up but I think the viideo should be full screen. Does anyone know how to fix this if there is a fix?
The second thing is that since I have this new tablet I want to give my brother...

A little help for a newbie

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