Relaxed body is going to generally activate your parasympathetic nervous system and relax your brain as well - here is a study showing that progressive muscle relaxation has a statistically significant effect: https://pmc.ncbi.nlm.nih.gov/articles/PMC8272667/

And I can guarantee you that you go into a theta state during sleep. Young children up to age of 7 spend a lot of time in a theta state, and as adults, we go there during REM sleep. It's considered to be a highly suggestible state, that's why it's used in hypnosis (more on theta: https://en.wikipedia.org/wiki/Theta_wave)

Hypnosis is nothing mystical like some people claim. It's literally just putting you into a deeply relaxed state so your brain is more willing to go with whatever is happening, but you still retain full control and can choose not to accept a suggestion.

When it comes to deeper hypnotic states and people claiming they couldn't control themselves and did what the hypnotist told them to - I cannot speak to that because I am not trained to do that, and I am not sure exactly how that works because I've never experienced it or witnessed it.

I do believe that some people are highly suggestible while others are not - and IMO it has to be with how easily they can go into that theta state. I've had clients who just couldn't get into it no matter what we tried - and it wasn't because of aphantasia, they just have a hard time relaxing in general and are concernced about what is going to happen under hypnosis. It's not for them, and that's fine.

I don't believe that there is a single therapy approach out there that will work for everyone - although purists will definitely claim their approach is the best.