Who determines what to and what not to follow within the religion?

There’s some verses in the Talmud that are adhorent, I won’t list them out of respect and because listing them isn’t very relevant to my question but who is it that decides wether a Jew should follow something in the Talmud or not?

I understand in the Talmud there’s contradicting verses but how is it determined what a Jew should and shouldn’t follow and who determines that and why is it they who determines that? Or is it a free for all and anyone can interpret it however they feel is most correct?

Also I understand polygamy isn’t practiced anymore by Jews, this (afaik) is because a rabbi said not to therefore I question why does a rabbi have authority to change religious law? Why do you respect it and what’s stopping another rabbi popping up and changing the law even further?

Saying something like “the rabbis who put that in the talmud were oppressed or from a different time” isn’t an excuse, therefore either the whole talmud has legitimacy within Judaism or there’s some scoring system that I don’t understand.