It’s not a mobile client sadly, but for desktop I’ve always liked “4k video downloader” (Softonic). It’s something you have to download on there, but I copy the youtube link, open the app have it parse the link and then it’s downloaded and ready in .mp4 pretty quickly, with .mp3 as an option too for music.
It’s a bit convoluted, but I’ll usually download albums and tracks on my PC, have it all saved in a music folder then copy it manually to my phone.
Honestly if it’s a big setup like Spotify or Netflix or etc, “royalties” don’t mean shit to the production team / artists / whoever. Unless they’re Mariah Carey levels of replayed every year everywhere to the tune of sitting on millions in checks yearly, they aren’t going to get shit. Personally I’d rather support them in other ways such as buying merchandise, going to live shows etc but that’s just me.
Remember though this is in minecraft, don’t pirate irl because that is very bad and you will personally prevent executives at warn-a-brother from buying another learjet. Remember: “pirating is bad and you should feel bad” 😂