Calculate the calories your body burns at complete rest using the Mifflin-St Jeor formula, the most accurate according to studies.
It's the calories your body needs at complete rest for vital functions: breathing, circulation, cell repair, etc.
Yes. Gaining muscle mass (each kg of muscle burns ~13 kcal/day extra), sleeping well (7-9h), staying hydrated and avoiding very restrictive diets.
No. BMR is at rest. TDEE includes your daily physical activity and is always greater than BMR.