I just wanted to cut a few Minecraft Mondays to practice, and of course music shouldn't be missing. The question now arose whether I can even download music from YT in order to use it for practice (not for uploading to YT or something)?
Yes it is allowed, but only for private use
So if you don't publish it then you can.
At least this function is not provided on YouTube, so I wouldn't brag about it in front of the police. But there are programs for that, and normally nothing happens to you.
So there are websites to download that really work. Whether that's legal is the other question. If I were you I would just do it and then show it to nobody.
YouTube's terms and conditions do not allow it. Everything should be said with this.