I'd also add that, generally speaking, even a third person omniscient narrator tends to only go into the head of the protaganist, not everyone.
No, the general pattern in modern third person omniscient writing is that you can write from the viewpoint of any character in the story, but it's preferred to focus on only one character's viewpoint per scene. In older books, it wasn't uncommon to jump around between different characters' internal monologues within a single scene, but the modern standard is that if you want to switch to a different character's point of view, you need to put in a scene break or a chapter break before doing so. For instance, I might write a scene from Kirk's perspective on the bridge of the Enterprise, then cut to a scene from Scotty's perspective down in engineering, then cut to a scene from Spock's perspective leading a landing party. And if I then want to switch to McCoy's perspective on the same landing party to show what he's thinking about Spock, I still need to put in a scene break and do the next segment strictly from McCoy's perspective.
One of the choices I generally have to mull over before writing a scene is whose POV I should write it from. It's generally best to go with the person who has the most emotional stake in the events being described, the one who has the most to lose. Although if one of your characters has some deep dark secret, like they're the murderer or they're an impostor or they're an enemy pretending to be an ally, then you want to stay out of their head until the secret comes out -- which runs the risk of making it too obvious that they're hiding something.
Sometimes a story stays mostly within a single character's head even in omniscient third, but that's generally for shorter works. It can be good for a novel that's centered on a single lead, but if it's more of an ensemble story, then it's better to jump around among various perspectives.