The difference is build quality.
When your bike weighs 30+ lbs and is gonna go 28mph you want good hydraulic disc brakes.
Does it have suspension? There's a massive difference between shitty springs and nice pneumatic.
The quality of the shifting. How nice and snappy will it shift? And will it stay like that? Or cause you to swear as your chain skips and jumps off causing you to eat shit.
Frame? How well is it made? Can it take years of gradual abuse?
Everything else? How's the headset? On a shitty 500 dollar e-bike it's probably gonna be a mess.
The electronics for the interface? Like the display? Sure how many features does it have, but is it built like a shitty 10 dollar casio watch and is it gonna break in 3 months?
an e-bike under 1000 dollars sounds like a waste of money. It's gonna fall apart and you'll hate it.