That's a lot of questions. T's do learn a great deal of theory: developmental, psychological, cognitive, etc. And yes, it is very important that they do so. Whether that makes them "better" or not is a whole other matter.

T's go through internships and supervision during grad school and often beyond.

T's learn various therapy techniques, so yes, some of their response is because of what they have learned, but hopefully they are well beyond using a script.

T's should be pretty good at picking up on body language. They generally are pretty good at having a feel for what we are feeling or thinking, if that makes any sense, but they can't read minds, so don't expect them to. It's all only an educated guess until you are open with them.

I don't think they think it is a one size fits all, but there are tendencies that are somewhat predictable in clients with similar experiences.

What makes t work or fail? That's the big question. I think (a) it takes a really good therapist (b) it takes a client who is ready to do the work, and (c) the two have to mesh well enough to create a safe, honest, and trusting environment.