NaOH turns fat into soap.

It’s dangerous because you’re also partially made of significant amounts of fats (and other things), which gets very very painful and can make it somewhat difficult to continue living when they get turned to soap while still part of you.

Separate from it's use in the production of soap, it's used as a cleaner (often at very low concentrations).

In the production of soap, it is used to break down the fats used rather than being part of the soap.

https://en.wikipedia.org/wiki/Saponification

It's a precursor but there's barely any left of it in the final product.

NaOH is mostly dangerous because it is so alkaline. When reacted with fat to create soap the alkaline OH- is reacts with the fat and is 'used up', leaving a mostly neutral product

Yes - kinda like (horribly poisonous) chlorine gas is part of table salt.