My height is around 174.8 cm and weight is around 55 Kgs.

The problem is, I wasn't under weight when I was around 14-15 years old. Now I'm 26 years old. I never paid attention to it until after entering 20s.

I guess the possible reason for this is my stress about life, loneliness, thinking too much about future and not eating much. I maybe wrong though.

Now, I read many answers regarding weight gain here.

Most of them ask: Do you need muscle gain/build muscle or just weight/fat?

Thing is, I look weak. My legs and hands are thin. My cheeks are concave (I don't know exact word for it. I mean cheeks are pushed inside unlike of most people. Hope you understand it). Sometimes when strong wind is blowing and I'm walking, I feel the force is pushing me.

So I don't understand when you ask muscle gain or weight gain or fat etc.

I just know that I'm not fit in terms of weight and I want to be fit/normal.

So Q.1 To be fit in terms of weight, (so I don't look skinny and become like other people of my age), do I need muscles or fat or something else? (I need answer of it so I can proceed further)

Q.2 For some reasons, I can't join gym and can't buy equipment. I can only eat and do exercises without equipment in home. Does squat work to gain weight? What is proper way to do squat? Is it different from sit down and get up thing?

If you need more information, kindly mention in the comments.

PS: Which answers can I read on this website to gain weight, once I understand how should I proceed?