How exactly does a fitness tracker calculate the calorie consumption?

I bought a fitness tracker. It is the Huawei Band 3 pro.

I train regularly for about 2 months on the crosstrainer and do strength training. I have the fitness tracker now once with a unit (30 min intensive crosstrainer + 45 min strength training) run. The thing shows me now almost 900 burned Kcal. (Incidentally, I'm 1.90 tall and have a very clear overweight).

Now I've just loaded an app that calculates the calorie consumption, there I indicate size, weight and gender and wear a how many minutes I have trained what. The result is 480 kcal.

Now I'm just interested in which trend is more appropriate and how the tracker comes to its value. And above all, how accurate this tendency is.

Probably someone knows the device and has idea of the whole matter and can give me a good information.

It actually applies: sport-dependent factor in kcal / (kg * hour) * body weight * exercise duration = calorie consumption. The tracker would need to know your weight and know what exercise you did for how long.

No matter what tracker it is, no one can give you any information no matter what.