That depends on if the YT video uploaded by the source was actually encoded 5.1, because most YT videos are stereo due to the source files being multi channel in a format like LPCM (where YTs encoders cannot process it direct in 5.1) and always converts it down to AAC 2.0. If the source video file has an audio codec YT accepts (like AAC which can go and be set up to 5.1) it’s usually left intact and most video downloaders can detect and grab it with the video stream.

If the video is indeed in native 5.1 but the downloaders cannot grab it or the correct audio, worst case scenario is configurating Windows/Realtek sound driver to output 5.1 then setting Audacity to have 6 audio busses and record in Windows WASAPI mode.