@Spectre660 is right.
All the youtube videos have 360p and 720p one file streams, which are playable using mplayer. Live videos and Twitch or Mixer videos are all m3u8 files with small chunks of videos, usually 2-5sec each. Those files are playable with mplayer, but it stops at the end of each one, and then starts the next one. Also, it doesn't update the playlist and stops when this ends. This behavior exists even on Linux/MacOS versions of mplayer.

The solution for these streams is to port ffmpeg and ffplay. I worked on all these stuff because I created the aiostreams scripts that can be used on AmigaOS 4 and MorphOS, among other systems, but MorphOS lacks currently a player to support them.

Have a look at:
https://github.com/walkero-gr/aiostreams
https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=12689&forum=9#142698