Btw, what is up to gstreamer vs. MPV vs. ffmpeg ? I dont get the difference.
As far as I understand, MPV is a video player application and offers libmpv as a high-level video playback library. I'm not sure, if it makes use of GStreamer or ffmpeg under the hood, but it wouldn't surprise me.
GStreamer is somewhat lower in the stack and much more generic. It allows orchestrating various steps in a media processing pipeline, so could also be used for video editing or transcoding or the like.
And ffmpeg is a library specifically for transcoding of as many formats as possible. You can use ffmpeg as part of GStreamer via a plugin, for example. But there is also a really useful ffmpeg CLI, with which you can script all kinds of file conversions, like MP3 to OGG Opus is something I do often.