It's easier to understand common nouns by contrasting them with proper nouns. Proper nouns are specific people, places, or things, while common nouns are more general people, places, or things. For example, "Hiroshima" is a proper noun, while "city" is a common noun. Based off this explanation, the common nouns in the above sentence are "morning," "bomber," "bomb," "city," "people," "city" (again), and "blast."
That depends on the situation, if he is acatully going somewhere then he is literally getting or buying some milk, if he is still around thaty means he sai that with a expression or metaphor