It's a cult. The "science" they base their beliefs on is called "dianetics". You can look that up and get more straigh-up explanations than by looking up "scientology".

In short, they think humans are possessed by the dead souls of immortal aliens from millions of years ago, but we can't perceive this due to traumatic memories which must be "cleared" using "auditing".

Auditing sessions are recorded, and as they involve confessing your darkest secrets to "clear" the relevant "trauma", the recordings can then be used to blackmail people into staying with the cult.

It's a pay-to-level-up religion, except instead of caring for your health they abuse you. They actively reject mental healthcare based on real science, and consider psychiatrists equivalent to murderers.

They don't believe in the concept of crime, instead considering anything and everything that happens to someone their own fault.

Members are not allowed to report crimes perpetrated by other members to the actual police, instead they must be reported to the church. When scientologists rape other scientologists, the victim gets punished with more auditing.

The most infamous scientologist is likely Danny Masterson, who is finally in prison for assaulting likely dozens and dozens of female members.

They also don't tell their followers what their beliefs actually are, before they've paid so much money for it that the sunk cost fallacy has them too committed to pull out.

You can find more info online about their actual beliefs told by people who have left the cult, than they reveal even to their own followers. Its all deliberately confusing, because no-one would buy into their crazy bullshit otherwise.

Get your daughter out of this relationship asap. Or even better, have a serious talk with her about scientology, explore what it is and what it does to its members, together, so she can then consider the situation and navigate it for herself.