Usually I recommend using cobalt.tools for downloading either the video or the audio file. Unfortunately, cobalt.tools isn’t able to download from Youtube at the moment due to restrictions from Youtubes side.

I found a workaround, albeit it is a bit complicated, but suitable for an occasional download: I installed FreeTube. Each video offers download options. However, if you want a better quality than 360p, it offers to download video and audio separately. Look for the highest quality. Both video and audio file can be merged using VLC player after downloading.

After selecting the desired file(s) in FreeTube, you deternine where the files are to be saved, and then the download starts. There is no visual confirmation that the file is being downloaded. Eventually it appears on your computer. The download is slow, it takes almost the duration of the video that is to be downloaded.

After downloading both the video and the audio file, open them in VLC. Follow one of the guides when searching for video audio merge vlc. You can also save the merged file.