Biological sex determines the BMR constant in the Mifflin-St Jeor equation.
Choose the level that best matches your average week, including non-gym movement like walking and manual work.
Mifflin-St Jeor works for most people. Katch-McArdle is more accurate if you know your body-fat percentage because it uses lean mass directly.
A 500 kcal/day deficit produces roughly 0.5 kg (1 lb) of fat loss per week. A 1000 kcal deficit is aggressive and not recommended without medical supervision.