The problem with the "free" music library is that the licensing from the music owner can change at any time, maybe tomorrow, maybe next year, then youtube has no more rights over the tracks they let you use for free and the new owner of the rights can claim copyright infringement on the video you used the music in.
Youtube doesn't care and don't give a f+++ about small channels. I ended up deleting the video in question because it was stuck, couldn't play it back or download or change the audio or anything. The only options I had was aknowledge the claim or appeal and I couldn't appeal because youtube wanted my cell phone number.
When you monetize your channel, you get a slice of ad money for showing them into your material. If there's a claim on a video, the copyright holder gets all the money from the advertising and your video remains online but with ads even if you like it or not and you get no money from your material.
If you don't make money with your videos, you get a claim and ads are placed the same way, like it or not and again you get no slice of the cake.
If you complain and fight back, youtube can give you a copyright strike and when you get 3, your account is closed.
Some copyright holders are denser than others, they may, at their choice, put their ads on your vid an make money or ask youtube to take your video or account down.
Copyright on youtube is a very complicated matter, if you don't care about losing your channel or if you want to make money with your material, make sure you don't get even close to any copyrighted stuff. Any problem with youtube and the ridiculous content ID and they immediately stop the cash flow to your account. Solving such problems can take months and in that time you get no money even if your video is still online and receiving ad clicks.