It's just the difference of who's telling the story. The narrator is seperate from the character in the same way that I would be if I told a story about my friend -- which is what third person is. So if I were to tell a true story about my friend, I could embelish by adding something they didn't notice at the time. For instance, they were standing on a rug that they didn't know was covering a coffee stain. I am seperate from my friend's experience, thus I can tell the story how I want. I would be the narrator.
I used to write in third person only telling what the POV character noticed (somewhere slightly less than twenty novels). I started doing research and reading other books and I saw that you can do a lot more with third person than that and I was able to diversify my writing. Now I write by choice in first because I think it's fascinating see the story solely from the main character's flawed mind, especially when the main character is either crazy or going mad.
Nope when you're in 3rd POV you only know what that character knows and notices. The narrator is not separate from the character. For example, if the MC in 3rd POV has their eyes closed you can't describe what's happening in the room because they're not seeing it.
If the narrator knows things the MC doesn't then it's an omniscient narrator, and yes they can move from one character to another from one scene to chapter to another.