> At what point is a company no longer a startup and just a small business looking for talent?
I'd say when it stops accepting funding and determines it wants to stay as a small business. But that's probably a bit rare for companies that have taken on significant funding. After all, if I'm an investor I'd like my money multiplied (>1, thanks).
> Are typical start ups really doing whiteboard interviews during their early stages?
It varies a lot on the startup and the founders. Some founders don't have a large network of engineers (or - rather - engineers who would like to work at a startup for startup compensation; this is especially true with very early startups). You'll eventually have to hire from outside because your network will run out.
I've been hired as an early engineer at a seed stage startup. (Would not recommend) I've interviewed at dozens and dozens of startups in SV all across the funding range. Many of the ones I interviewed with did ask typical whiteboarding questions that you'd find at FAANG. After all, many come from FAANG and don't know any other way to interview. Or they try to imitate "the best".