Does the deceased person know what is happening to his/her body while it is being given ghusl, buried, etc.?

Answer

Yes, the deceased person knows, sees and hears everything that is taking place while he/she is being washed, clothed, and buried. Rasulullah (sallallahu alayhi wasallam) said that the wails and groans of the evil deceased are heard by all creation but man. If people had to hear the wailing of the dead, they would fall unconscious.