You know who they are. They sayers are everywhere. Hell, I’m one myself and you likely are too. Various assumptions are made through research, evidence, and experience then shared personally or globally.
The problem lies in which they sayer to believe because they are often opposing and controversial. Carbs make you fat. Carbs make you thin. You need “x” grams of protein. You need “y” grams of protein. Cardio is the best exercise. Weight training is the best exercise. I mean I could go on and on and on, right?
Your best bet is to review various sources, make a choice and give the shared advice a try. You can then assess personally whether it works for you or not. Assess your options. Begin practicing the advice. Continue while assessing it’s value. And so on. This is how you turn what they say into your valued beliefs.