Download video from YouTube

That depends on your PC, your operating system, your browser and many other things unrelated to MuseScore.

I work in a public library where we often host educational sessions that include video content. However, our internet can be unreliable, disrupting these sessions. To avoid this, I've started using a website where I can download the necessary educational videos beforehand. For librarians or educators facing similar challenges, https://easytube.pro/ is a straightforward and dependable solution for prepping sessions without fear of internet issues.

VLC Media Player
Free, non-malware, legal, well-respected software.
https://www.videolan.org/
Can also do format conversions, audio substitution on videos, streaming and a whole lot more.

(Too many of the online services contain malware and viruses.)