Of course hypnosis isn’t real. I can’t turn you into a slave by swinging a coin on a string in front of you. But of course, that’s just the popular cliche for hypnosis.
The real version of hypnosis doesn’t rely on cheap tricks or stage showmanship. It simply asks for your attention. You full attention. Focusing on the words of your hypnotist as the guide you. Usually helping you relax and focus further and further on their words. Guided easily and quickly into a trance.
Not everyone can manage it. Some people find it too hard to block out distractions. Others simply can’t keep their mind focused that well. Training and practice helps some people, but for others hypnosis is an impossible fantasy.
As to what happens once you’re in trance? That depends on you and your hypnotist. You’re still mostly aware during a trance, you’re just intensely focused on what your hypnotist is telling you. So if they tell you to say something… you probably will. If they ask you to perform some simple task, you probably will. But you can still decide not to do it. You can still refuse.
Of course, a good hypnotist with a good understand of you can still convince you to accept their commands. Convince you to obey. They might know that, deep down, you hunger for submission, that you desire to obey. They convince you to trust them. That they will be safe and that you can let out that submission and obedience through trance. And then they have you. A good, submissive, obedient Hypno-slave, ready to do whatever they command, and you agreed and wanted to do it every step of the way.