You don't need to know all of these things if you are measuring input (calories in) and your output (weight on the scale every week). It hardly matters then how efficient your gut is. These are all relative values. Of course 2000 calories will look different for different people. What matters is if you have an understanding of how much calories you are putting into your system, you can now make changes and measure a response, like cutting down some calories and seeing how that affects your weight over a few weeks or months. You don't need to know all of these rates to do that.