Speaker A: I'll go buy some food at 7-Eleven.
Speaker B: Wait what about those/the one I bought? (sushi, rice balls, and instant noodles)
In this case, should food be considered a single item (as in the food)? Or many items (sushi, rice balls, and instant noodles)?
Or maybe it doesn't make much difference?
Food is not countable when the meaning is sustenance. It is countable only when referring to particular edibles as types or categories, or as elements of a diet, not as sustenance per se:
She is allergic to certain foods: nuts, eggs, and shellfish.
Omnivores are not very selective about the foods they eat. They will eat almost anything.
Sushi, a food that originated in Asia, has become very popular in the West.
To survive we must have food.
For that reason, the following reply is ungrammatical:
Speaker B: Wait what about those/the one I bought? (sushi, rice balls, and instant noodles)
A grammatical reply would be:
Wait, I've already bought some food.
Wait, I've already bought some.
Wait, I've already bought something to eat.
Wait, I've already bought something for us to eat.
And if we wanted something closer to your original question:
Wait, what about what I bought?
{what about} {what I bought}?
Tᴚoɯɐuo had a solid answer I think, but I can expand on it.
Food is a collective noun in its own right, it describes any amount of edible items without limit.
If you attach "The" to it, it refers to a specific volume of food. Eg: "The food" that I bought
Going to 7-11 to get "some food" is non-specific, it could be anything from a single apple to a three-course meal.
So referring to it has to be non-specific too. Even if it's oblique as it is in your second case.
You're referring to a very specific instance of food, the meal you already bought. As opposed to the more hypothetical "some food" that the first speaker is hoping to get.
So the most grammatical phrasing for your second case that I can see would be:
"But what about the food I bought?"
You could alternately refer directly to the food items you bought, eg:
"But what about the sushi I bought?"
but in the interest of your question I think the first suggestion is more helpful.
