When it comes to dieting, you can't leave out either exercise or diet, right?

I am planning to go on a diet, and I am wondering if I absolutely have to do both exercise and diet.

I've heard that some people lost weight just by controlling their diet, while others said their bodies improved just by exercising consistently.

Ultimately, I am confused about whether diet is more important for weight loss or if exercise is absolutely necessary.

If you are short on time and have to choose only one of the two, which one should you prioritize?

I am curious if there is a big difference between doing exercise and diet together versus doing only one.