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.