Knowledge of cooking was passed down from generation to generation via schools, guilds and families. We stand on the shoulders of giants. Thousands of years of trial and error passed down. People measure by sight, sound, touch and smell. For example, there are multiple ways to guess steak doneness. Simmering is visually different from boiling which is different from a rolling boil. If you're frying something in a pan, you can guess the temperature by sound. If you're making a stew or braise, it's done when the meat breaks apart. Doesn't matter what the cookbook says about exact times because environment and cooking equipment is different for everybody. Water boils before 100 C at high elevation. I moved to an area with high humidity in the summers and all my baking recipes from home failed.
With practice, you also build an intuitive sense of doneness. For example, I've cooked enough "black on the outside, raw on the inside" chicken to know how hot the heat should be and how long it should take--even without a thermometer or a timer. Like others have mentioned, sometimes you just have to cook a proxy item or sacrifice a piece.
One of the big differences between new cooks, and experienced cooks is that people who have cooked for a while are constantly tasting, touching and inspecting their food. New cooks tend to follow recipes word for word and only taste at the end. Then they get surprised when something is under/over cooked and under/over seasoned.
Lastly, I think previous generations had different expectations of consistency and quality. Modern society is hyper-precise. Traditional recipes have a huge margin for error.