Jon,
If you’ve been reading the forum, just the other day Sergey (former ATP student and instructor) got hired with NetJets (A journey of a thousand miles starts with a single step). It’s very different flying and there are pros and cons to both.
As far as the path there is no requirement to fly for an airline first. As with any job you need to meet the experience requirements. Thing is the Regionals need pilots bad so it’s an easy gig to get and they also offer the opportunity fly alot and build time fast. So while it’s not a requirement, it is a route many pilots take to get the “better” corporate jobs. There are of course entry level corporate jobs but they are harder to find.
Adam