For me on my S10+, when watching videos on YT and turn it into landscape it automatically shows top to bottom of the screen, then pinching out expands the video to use the whole screen and yes this results in some loss. I don't believe there is any other way, you either have the full video the aspect ratio that it is or force it into the ratio of the screen. I guess you could try to find videos that are the same ratio as the screen but otherwise you would end up with squished videos where everyone's head and body was short and fat.