How can different cell types form if they have the same DNA?

This can happen through the process of differentiation by when an lets say a stem cell can differentiate into other types of cells and specialise to do a specific role/ take on a specific form for example red blood cells.

essentially, one bacterial cell splits into two daugurer cells. bacterial offspring don't need or get care from their parents because they are in a sense their own parents. that said, some bacteria can and do depend on other bacteria especially in harsh environments such as bacterial microfilms in hot water tanks.