I googled this question to refresh my memory. Google’s terms of service have this:
“You are not allowed to…access, reproduce, download, distribute, transmit, broadcast, display, sell, license, alter, modify or otherwise use any part of the Service or any Content except: (a) as expressly authorized by the Service; or (b) with prior written permission from YouTube and, if applicable, the respective rights holders.”
So personal use of the video you described sounds okay, but you need software and since most downloads aren’t legal I have found software I used in the past to be filled with suspicious attempts to do bad stuff.
I consider the article I found to be complete and bone fide. If you want to use its suggestions here it is with lists of software you could try. [But do be careful.]