It’s also just really hard unless you live off packaged meals or only eat thing that are isolated.
Something like a curry cooked in kitchen and shared among a family is a complete black box as to who got how many calories. Maybe one person got a different ratio of rice to curry. Or this family likes a sweeter type of curry etc
For an individual trying to lose weight this isn't a problem - if you are not losing then you just need to eat less. For population level trying to figure out if curry is healthy in the first place this matters though (is it curry itself that is good/bad, one of the spices, or how much sweetener added - if all we know is curry that isn't helpful)
No, you get a food scale and weight every single ingredient before adding it to the meal. As for who got how many calories, weighing of the portions should provide that information given the ingredients and how much of each there is.
Your metabolism is a system. Like any system, data about its inputs and outputs can be gathered if you would but measure it. Make getting accurate portion sizes a part of your daily routine.