BMR (Basal Metabolic Rate) is what your body burns at complete rest — breathing, circulation, temperature regulation. The Mifflin-St Jeor equation (1990) estimates it from weight, height, age and sex: 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 for men, the same with −161 instead of +5 for women. Across a 1990 validation study of 498 people, it predicted measured BMR within 10% about 82% of the time — the best of the four major formulas compared, and the reason it's the default in most clinical settings today. Multiply BMR by an activity multiplier (1.2 to 1.9) to get TDEE — the calories you actually burn in a day, the number that matters for any weight goal.
A 30-year-old man at 70 kg and 175 cm has a BMR of 1,649 kcal. Sedentary, his TDEE is 1,979 kcal; lightly active, 2,267 kcal — a 288 kcal difference from activity level alone, more than most single food choices.
Women carry proportionally more body fat and less metabolically active lean mass than men at the same weight and height, which lowers resting energy needs — the constant is simply what the original regression found fit the measured data, not a rounded assumption.
Because most people count workout intensity, not weekly total — three hard gym sessions plus a sedentary desk job the other four days is "lightly active," not "very active." Overestimating activity is the most common reason a TDEE estimate ends up too high.
Mifflin-St Jeor is typically accurate to within about 10% for most people — meaningful, but not exact. Treat the result as a starting point: eat at the target for 2-3 weeks, track actual weight change, and adjust by roughly 100 kcal/day if real results don't match the prediction.
Only if your chosen activity level didn't already include that exercise. If you picked "sedentary" and added a workout separately, eating back some of it is reasonable — but if you picked "moderately active" because you already train regularly, that training is already priced into the multiplier.