ELI5 Where do fruit flies come from?

My daughter left a half eaten apple at the bottom of the bin in her bedroom at some point during this week- it can’t have been there longer than a few days because I only emptied it on Monday evening. Now she’s managed to attract loads of fruit flies.

*Yes I am aware it’s gross and no, my kid definitely should not be leaving food to rot in her room. She’s now very much regretting that decision.*

I haven’t found and didn’t previously spot maggots or eggs etc anywhere in her room when I’ve been in there cleaning. It’s only the one room that they seem interested in- the rest of my house is reasonably hygienic (thanks mil!) and fly free - they have only been hanging around that bin.

This has me wondering, whilst I’m dousing every inch of my house with enough fly killer to put another hole in the ozone layer, where on earth they come from?

They seem to appear out of thin air and; you don’t see any eggs or maggots anywhere; when they arrive there’s never just one, they always come with about 9000 friends.

They obviously don’t just fly in to existence through some kind of inter dimensional rift, so where were they hidden before they started infesting my kids trash?

Do they live outdoors usually? Are they reproducing insanely fast (2/3 days) or do they just hang around in flocks and hunt for smelly fruit- how do so many appear so fast?