Censorship is a problem when you are not free to express an idea.
The form of the idea can be limited without it being censorship: for example, you can't protest with a bullhorn in the middle of the night.
No, you're confusing the definition of censorship with what restraints on free speech are constitutional. You're defining the term based on legality in a specific jurisdiction. If you applied the same standard with a different jurisdiction — say what can legally be spoken in Iran — you would get a very different definition. This is why you cannot define censorship based on a specific legal standard. It must be defined in the abstract.
The general definition is that censorship is the suppression of expression by an authority. That expression could be a constitutionally protected idea, or it could be an advertisement for Taco Bell. It's still expression, and blocking it is still censorship.
Free speech doesn't mean everyone has to listen to you, it means the people who want to listen to you are able to. Youtube demonetizing a video is not censorship. Removing videos because they don't like the ideas in the videos is the worst kind of censorship, because they are trying to control you.
And again, you're using the U.S. definition of free speech to define censorship. These things are not synonymous. Nobody is talking about whether someone should have to listen to you. The fact that someone shut you up is still censorship. It may be *constitutionally allowable* censorship. It may even be *entirely* *reasonable* censorship. But it is still censorship.
Not all censorship is bad. Restraints on speech based on time, place, and manner restrictions can be entirely reasonable. They are still, at least in a purely abstract way, a form of censorship. Restraints on speech where your speech costs someone else money can also be entirely reasonable (e.g. spam, robodialers, etc.). But those decisions still represent censorship in the abstract. We simply choose to say that those types of restraints on speech are reasonable and necessary for a functioning society.
It's all about balancing rights — your right to swing your fist ends when it meets my nose, and all that. Censorship is generally justifiable when exercising your right to speech has the effect of violating someone else's rights. For example, if you call for violence and that violence happens, you can get thrown in jail, and your rights to free speech can be massively curtailed. This is, however, still censorship in a purely abstract sense.