I’ve seen this equation in help menu (default NDVI):
NDVI = (IR_factor * near_IR - red_factor * red) / (IR_factor * near_IR + red_factor * red).
Can you tell me more about those factors?

I want to calculate some other indices and because of that I’m interested if I am supossed to use those factors and how? Also I would like to know how to calculate them.

Google says NDVI= (nir-red)/(nir+red). When I use this formula, I get one values and when I use default NDVI I get different values.

For example I want to calculate EVI=(2.5NIR−RED)/(NIR+6RED−7.5BLUE+1). Do I need red_factor? If yes, what is it value?

These factors in the EVI are not related to our factors.
The IR and the red factor are intended to make the NDVI result of two different sensors comparable.
For example these factors can be derived from the spectral responds functions.

Thank you sir for response!
However I’m not sure how much I understood. I am student and also not native English speaker. Correct me if I am wrong. I can ignore factors when I am calculating bends and making indexes by myself. I am only using imagines from Sentinel 2 sensor. It would be much easier for me to understand if you could use examples as explanations.