If you are using American English they are all garbage, not garbages. However if you want to use UK English you would say they are all rubbish. A bottle becomes rubbish after you drink the soda. The tissue, after use, becomes rubbish. The milk carton, after use, becomes rubbish. All together they are rubbish or it is the rubbish. The word garbage is not commonly used in UK English.
"they" are plural, but "garbage" or "rubbish" is uncountable. So I was afraid I would make a mistake by putting the two words together. Your answer has cleared up my doubt.