PC Magazine shares resources for creating “personal use” only legal downloads from U-Tube as noted in links below.  One should respect artist rights & all legal copyright considerations when creating MP3 and MP4 formats

https://www.pcmag.com/news/331905/how-to-download-youtube-videos

https://www.pcmag.com/feature/362854/how-to-convert-youtube-videos-to-mp3-files

When the topic of downloading YouTube videos comes up, there’s a side subject that must be broached: Is it legal?  On the copyright front, as long as you’re downloading a video for your own personal offline use, you’re probably okay. It’s more black and white when you consider Google’s terms of service for YouTube, which reads: “You shall not download any Content unless you see a ‘download’ or similar link displayed by YouTube on the Service for that Content.”

After all, watching YouTube videos offline through unofficial channels takes money out of Google’s pocket and the wallets of the video creators. There’s a reason YouTube runs pre-roll ads: people make a living this way. The video below from Video Creators explains it; it references YouTube Red, which is now YouTube Premium, but the legal issues remain the same.