Depending on the content, you could pay for the proper licenses needed.
If the copyrighted material is in ContentID, oftentimes the owner won't issue a copyright strike, but instead do a 3rd party claim and monetize the video on their behalf. That way they receive any income generated by the video instead of the channel who uploaded it.
More than likely, this is what has happened to the content you've seen. Otherwise, it's just a matter of time before they're found and dealt with accordingly.