[Question] Copyright claims?

To start with: I'm not really well educated at all the copyright stuff, so I have little idea of how this works -_-

I'm streaming my guilds raids and some other games I play on YTG (and twitch in past), and while I do so I play music in background. I'm not huge, I have like 1 or 2 subscribers, and mostly ppl who watch these streams are my guildmates/friends, I don't do monetization of any sort. I used to do this on twitch and had 0 issues, but once I switched to YTG I started getting copyright notifications about music, with something about mandatory ads for copyright holders (wtf?) and even some tracks with country restrictions. I muted some of the tracks in my stream recordings that seemed to be the most harsh and left the rest, then saved video. In a few days I checked back - now I can't even find any copyright claims on those 2 stream recordings.

So what was that? Why these claims appeared? Where did they go? What can I do to avoid having to deal with them in future (besides not using music)? How does it work? What music is safe to use as background music and what is not?

EDIT: also I plan to stream on bigger channel in future (collaboration of several ppl, with monetization this time around), any tips for that?