If there is block A on another block B.

There are many forces acting between them. There is mg by A down and then mg by earth upwards.

Q1 why is mg of A not equal to Mg of A + Mg of B.

Then we have many normal reaction pairs. Force of earth on block B and Force of block B on earth.We also have force of block B on A and force of A on block B.

I have checked many videos that we have to consider forces that act on the body.

  The thing is  ,even on mass B .A force of Ag is acting.Who don’t we consider it  ?

I am getting confused because of Newton’s third law and how to make correct system and FBD for a body.No one on Internet has given correct information about the things that you do not include in FBD’S. It is getting really confusing.

I need help pls.

  I want to understand which forces are to be terminated when considering a system and making a FBD.

Consider the case where there is a block A on another block B.

There are many forces acting between them. There is $mg$ by A down and then $mg$ by Earth upwards.

Why is $mg$ of A not equal to $Mg$ of A plus $Mg$ of B?

Additionaly, we have many normal reaction pairs. The force of Earth on block B and the force of block B on Earth.We also have the force of block B on A and the force of A on block B.

I have checked many videos that we have to consider forces that act on the body. The thing is, even the weight of A is acting on B. Why don’t we consider it?

I am getting confused because of Newton’s third law and how to make correct system and FBD for a body. No one on Internet has given correct information about the things that you do not include in FBD’S. It is getting really confusing. I want to understand which forces are to be included when considering a system and making a FBD.