Let's say you bought a bunch of meat, cooked it, disinfected it, and put it away somewhere. It's 100% bacteria-free(which in reality, it's probably not). If you put it in a completely sealed, sterilized container, then it's not gonna rot. it might not taste good after a while since the protein, muscle, fat, water and stuff might not retain their original structure.

but we normally don't have that kind of container at home. if you put it in....let's say...a drawer, even if the meat is bacteria-free the drawer is probably not, even if it's, some bacteria might get in through the air or airborne water droplet or even walk themselves in there.

Even if it's just a few, they will eat your meat and make more of itself(using your meat they just ate), you won't notice at first, but the number of bacteria grows exponentially, and at some point, there will be so many bacteria eating away your meat that you might be able to see a few colonies of them.

basically, nothing is producing bacteria, they produce themselves using whatever they can eat(in this context, your meat), just a few individuals can turn into billions in a few days.

This idea applies to other microbes that grow on your food as well. like Fungi, other eukaryotes, etc.