Is dieting about diet or exercise?

I'm trying to start a diet, but I can't figure out where to start.
People around me say that diet accounts for 80% of the success, and that exercise is also important.
Some people say you lose weight just by exercising hard, while others say the opposite, so it's confusing.
Realistically speaking, I'm curious which has a greater impact.
I know I need to do both diet and exercise, but I'm curious about the proportions.
I would also like to hear how you felt based on your actual experience.
Please let me know if you have any recommendations for a beginner.