The fruit flies you've seen eating fresh fruit are probably Drosophila suzukii (spotted wing fruit fly). Most (all?) other Drosophila species (including the model organism Drosophila melanogaster) only feed on rotting fruit. Though they'll consume the sugars too, not just the microorganisms. So standard lab diets include sugar along with yeast and often cornmeal.

There are also some more distantly related flies that feed on fresh fruit and are commonly called fruit flies, eg Ceratitis capitata (Mediterranean fruit fly)

I’m sure the meme is apocryphal, and it’s not entirely correct. Fruit flies do eat yeast, yes…but they also eat the sugars in the fermenting fruit, not just the microorganisms causing fruit decay. I don’t know about the larvae at all, though.

There's even the old Marx brothers joke "time flies like an arrow; fruit flies like a banana"

The fruit fly trap taught to me by an entomologist friend is baited with wine, suggesting the fruit flies want fermenting fruit

The trap: get a plastic fizzy drink bottle; bigger is better

Cut off the top. The cut piece is a cone

Place the cone (the top piece) upside down on the rest of the bottle

Sticky tape the joint

Bait with cheap wine or leftover wine or beer. The bait is poured down the funnel into the trap

Her tastes were revealed in the description of baiting the trap — she specified red wine, but I have tested with various booze and fruit flies have no taste, they'll go for anything. You could probably extract bait from the sourdough starter you made in lockdown and kept alive despite never making bread after that first try