"Starvation mode" has been shown to be a myth. It comes down to basic math, calories in vs. calories out.
If you eat less and work out more, you lose weight. You do the opposite, you gain.
There's no way for the body to "magically" get fat when eating less. That violates the laws of thermodynamics. Sure, the rate of how quickly you gain or lose weight may change (e.g., when you eat less or change your macros to consume less sugar, you may find yourself being more lethargic in the short term until you get used to it, and so you will burn fewer calories). Or, as you lose weight, you need fewer calories (because there isn't as much of you to support).
But a calorie is a calorie and reducing ~3500 calories results in about 1lb of weight loss. Is it exactly 3500? No. Why? Because there are so many other variables at play. But is it closer to 3500 than, say, 500 or 10,000? You bet.