Three ways to reach the same deficit
A calorie deficit means energy intake is lower than energy expenditure, the condition required for fat loss over time. There are three routes to it: eat less (reduce intake), move more (increase expenditure through activity), or combine both. Arithmetically the deficit is identical regardless of route; a given gap between intake and expenditure produces a similar energy balance whether it comes from the food side or the activity side.
Why the routes differ in practice
Although the math converges, the routes differ physiologically and behaviorally. Cutting intake is usually the most reliable lever because exercise expenditure is harder to increase substantially and can stimulate compensatory hunger or reduced spontaneous movement. Activity, however, supports muscle retention, cardiometabolic health, and adherence. For most people a moderate combination is more sustainable than an extreme version of either.
This is general nutrition information, not individualized advice. Energy needs, health conditions, and appropriate rates of weight change vary, so anyone planning significant dietary change should consult a qualified professional.