I have seen cheaper and more expensive electric bikes from some hundreds to 10000$, what’s the difference?

Something with good specs but too cheap should be checked for availability of spare parts or accessible support because unlike regular bikes it really matters for an ebike owner.
Battery cells brand, depreciation or production date is a big deal. There is a calendar life on Li-Ion batteries; you don’t want to surprise yourself with a dead battery which costs 50% of the value after a year or so.
Li-Ion battery packs can be quite cheaper and more dangerous if not being manufactured under certain standards, qualifications or inspection. There have been many catastrophic battery explosions in Canada and US in past year due to poor battery pack quality supplied under the name of famous brands like Panasonic.
You might like to pay more on digital and display features, brand names, high end bicycle components especially for suspension mountain bikes; however the weight, durability, performance and the quality of motor/controller/battery will not vary much.