A short story is anything less than a novella. Since most novellas are approximately 20,000 to 40,000 words, a short story is anything less than that. The usual length for a short story back in the day when they were printed was 4,000 words because publishers allotted that much space in their magazines.
A novella used to be unusual because not many publishers had a place for them, wanting either a short story in a magazine or a full-length novel in book form.
A novel normally starts at 50,000 words and goes up to anything. 150,000 words isn’t uncommon these days because publishers want readers to think they are getting their money’s worth by giving them long novels.
I know authors who strive to shorten their books to a mere 100,000 words. I don’t and won’t ever write such long books but a lot of writers do.