i may be dumb, but can someone help explain calorie counting to me?

sorry if this is a dumb question, i'm autistic and have adhd so i can be kind of clueless at times. i never learned how to count calories. i want to start learning to cook and preparing my own food. i've heard things like to get a food scale and weigh each ingredient i use in food. do you just weigh the ingredients, then look at the calories on the nutritional label to calculate it? how would you calculate the calories in a finished meal? what would you do for something with no nutrition label on it? how would you calculate the calories in a portion if you were to prepare smth for multiple people, like if you measured all the ingredients and knew the calories for the entire dish, but what math would i have to do to know what 1 cup of that dish would amount to?

i feel like looking it up or using a database like mfp wouldn't be accurate bc if i got smth like, an apple for example, and if it had no nutrition label, well every apple can be a different shape or size even if they're the same kind of apple, like 2 golden delicious apples could be totally different. would it just be based on how much of the apple u use, like would 1 gram of golden delicious apple always the same amt of calories so using a database or looking it up would work in that case? ik people say it's not an exact science but that part confuses me too. idk i'm just confused how it comes so easily to other people but the concept confuses me sm and i've struggled to lose weight