It's very hit or miss. You're "suppose" to be allowed to use copyrighted material if you are making a parody or if it's for non profit, or other fair use cases. But Youtube does not see it that way. It's also random if, when and how they hit you. I get dinged on videos that I've had up for 5+ years sometimes. Most of my videos are in no way commercial, they arn't even monetized, but yet I still get hit. Sometimes it's almost instant. At times they just notify you but nothing happens, other times they mute the audio, sometimes they take the whole video down and give you a strike. I got a strike over a 5 second video once where they took it down. It was not even a copyrighted video, as in, not something commercially produced, it was just a clip from another youtube video. I've heard of people getting dinged for videos that they originally made and is 100% OC but perhaps someone uploaded it before.
It seems the bigger channels get more leeway and small non serious channels pretty much get a zero tolerance policy. I've seen clips of copyrighted work in other videos before on more popular channels, but I know as a fact if I tried that I would get dinged. Though there are ways around that for audio, such as slightly altering the speed. For video you can flip it, if it works for that context. Depends on the video sometimes it may simply not be an option to flip it.