Think of it like a piece of paper with information that is put into a folder. The folder is the object. The piece of paper are the data. The type of data(paper) put into the object(folder) determines what type of object it is. In the case of a mesh that is mesh data. Just like you have to open the folder to get to the paper, you have to enter edit mode to access the mesh data. So when you add in edit mode you’re just adding more data to the existing data. When you add in object mode you’re adding a new object with new data.
