How do the scene with the gravediggers and Hamlet's scene with Yorick's skull illustrate the broader theme of Hamlet's preoccupation with death?

It doesn't hamlet was trying to cheat death

The scene with the gravediggers illustrates the play’s broader theme of mortality. In the first part of the scene, two gravediggers discuss the burial of people who have taken their own lives and how the Christian system is flawed in disallowing suicide. Hamlet and Horatio then look at the remains of the many dead bodies and reflect on the certainty of death for all people. In death, we are all the same. For example, a woman may go to great ends to beautify herself in life, but her remains after death may look like any ordinary person’s remains. Hamlet and Horatio also discuss how a person's greatness ceases to matter when he or she dies. Hamlet refers to Alexander the Great being buried and becoming one with the sand.

Yorick’s skull acts as a symbol of death. With the skull in his hand, Hamlet reminisces about the time he spent with Yorick. Now, in death, Yorick is nothing more than a pile of bones, with no wit, humor, or intelligence. Earlier in the play, Hamlet spent much time mulling over death and wondering what came after death. Yorick’s skull answers that question for Hamlet.

The skull and the graveyard directly contrast with the life Hamlet led in the castle. In Elsinore, Hamlet’s mother and Claudius tried to make him forget about his father's death. In the graveyard, he has the freedom to contemplate death.