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.