Copyright on YouTube?

I started a YouTube channel and realized some of my videos were copyrighted: one had a song in the background and two were footage of a show I went to that had music. All three videos said they had matched the 3rd party content, but none of them were removed so I don't have any strikes on my channel.

I read up on what constitues copyright infringement and deleted the two show videos and changed the music on the other one, but there are still a few things I'm unsure about:

How is it that some YouTubers- ones that are for sure popular with viral videos and probably partnered as well- have things that are copyrighted? A lot of new/ politics channels show copyrighted photos from newspapers and the Associated Press and stuff, and others use clips from movies. TV or audio clips from popular music. I doubt every one of them have obtained permission from the producer. Not only that, many of them use clips from other videos- RayWilliamJohnson's entire channel is talking about, and showing, other viral videos. I guess he gets permission, but I'm not sure.

And that brings me to my last question: Where do parodies fall into all of this?

Many people have videos of themselves singing along to a popular song (think Call Me Maybe this past summer) or making a parody of it. Is this considered copyright? I know tons of people make parodies and they have become viral to the point that YouTube can't just be ignoring it.

In fact, there are tons of montages that have copyrighted music that get by, so I'm not even sure what to think. But back to the original points- are those copyright?

If you know about any of these, let me know!

Thanks!