We can't add or subtract from Torah. Torah and Talmud form the basis. When new situations arise, we look at that, and issue judgement. Those are called Responsa, and evolve over time. But there are strict rules on how one can come up with acceptable Responsa.Who sets these beliefs? There isn’t a central Jewish church.
When was the last major piece of scripture written? Who was the last prophet recognized by Jews?
Can anyone in modern times be considered a profit that can add to the holy texts? If not, why not?