The main difference is that bike shoes for flat pedals, vs. other non-bike shoes, is the semi rigid sole. A stiffer sole transfers more energy to the pedal rather than the heel collapsing as it does with non-bike shoes. But you won’t find cheap bike shoes.
When I started riding a mountain bike I used hiking shoes and so far they have been fine. I’m sure bike shoes would be better.
I use skate shoes for my flat pedal urban bike.
John