“I need to understand what those articles mean”
They are generalizing. It’s a true statement, just not an accurate one.
Either to keep it simple, or because they don’t know.
“so I could calculate my calories and protein intake correctly.”
For Protein, you want to consume 1.5-2.0 times your kg body weight in grams of protein. You can do more if you want, but the benefits of doing so aren’t clear.
As for calories in general? Closely monitor ALL of your calories for a week and make sure to eat the same amount every day. At the end of the week, weigh yourself. If you lost weight, add 500 calories and repeat the experiment. If you maintained, add 250 calories and repeat the experiment. If you gain too much, just cut back.
Your goal weight gain should be 0-0.5 lbs per week, if you’re gaining more than that, it’s probably fat. As a general rule, a human male can only build 1-2 lbs of muscle a month, while a female can build 0.5-1 lb a month.
For best results, plan out your meals ahead of time and utilize “MyFitnessPal”, as well as making sure that you follow a competent workout program. You’ll save yourself A LOT of time and misery by listening to someone who knows what they are doing. Yes, it can be pricey, but it is more than worth it. A program that I would suggest is “ATHLEAN-X”, but any reputable program will work.