"Het meisje leest over motors."
Translation:The girl reads about motors.
Words ending in -or can have two different plural forms: https://onzetaal.nl/taaladvies/advies/sponsors-sponsoren. So, "Het meisje leest over motoren" is also correct.
Engine does indeed usually refer to an internal combustion engine, but it doesn't have to. (Another example would be a steam engine.) Motor also usually refers to an electric motor. However, technically motor would be correct for a car's engine, hence the term "motorcar".
Technically I think motors have to produce some kind of mechanical motion, which is why the drives of fictional starships are called engines (warp engines, impulse engines) rather than motors (since their movement isn't mechanical).
In most real world uses however the distinction is purely that of fuel (petrol/gas, diesel, coal… ) vs not.
In general, yes. As Alphathon noted, 'engine' and 'motor' are used for combustion engines in cars and trucks, but 'engine' rarely refers to an electric motor. (Also, 'motor' is rarely used for the engines in ships, large aircraft, or train locomotives but is used for motorcycles, lawn mowers, and small aircraft; there's a size component to whether or not it's used as an alternative to 'engine'.)
I'll sign off on all of what Alphathon and adamskj wrote. And I'll add that the retractable sunroof in my car has a motor in it, but not an engine. We also have search engines, siege engines (back to ancient Rome and the Middle Ages, including ballistae, mechanized battering rams, towers, catapults, and trebuchets), and several engines (without physically moving parts) in computer science. And none of those is a motor.