The important aspect to understand when gaining weight is to consume more energy (calories) than burning fat. As with weight loss gaining a healthy weight requires some time and patient to achieved it. Healthy weight gain of 1-2 pounds per week can be expected when there is an increased amount of energy intake. The goal is to add an appropriate amount of muscle mass not only fat. Resistance exercise is also important to increase muscle mass, eating excess calories or protein alone will not increase muscle strength or size. The important thing is to make sure all your calorie choices are nutrient -rich and calories dense, you need the nutrient power of all the food groups that you will need to gain the weight you want.

https://healthwatchline.com/2023/05/16/healthy-and-effective-ways-to-gain-weight/