Short answer: Youtube is owned by google, and Musi is available in the google store. So, it’s fine.
Long answer:
Xanthippe_Voorhees:
Youtube doesn’t seem to be particularly against apps like Musi and they are so large that if an app violated their TOS they’d shut it down quicker than you could blink.
Yes, but…
Yes, youtube manage their licence well enough for you to trust that if something (eg. Musi) works, then it’s OK with youtube. You don’t have to go into the fine print yourself.
However, there are some exceptions, but with the current state of the web you are unlikely to stumble on them accidentally. Particularly, you can trust anything from the Apple or Google store.
In the past there have been browser add-ons which rip content from youtube for a user to keep off-line. Such are effectively stealing and immoral to use, because they bypass the sites legitimate control over its own content. Youtube could not block these because the add-on was running in the client and was undetectable by youtube. With the current state of web technology and legal enforcement I don’t know whether these add-ons are still available. A google search won’t find them, but they may be passed around on the dark web.
Today, with the dominance of smartphones, when any app is accepted by the Apple or Android store it is reviewed for its legality. The stores do not accept apps which are intended to breach copyright. Moreover, as you indicated, sites such as youtube have such clout that they could block any app they don’t like.