Thanks to both of you.
Now a follow up question:
Part 121 ops means all "normal" airline ops, including typical farmed out regional airlines of major carriers with CRJ's etc, right?
Part 135 is then typical business aviation, where as a customer you hire a provider to fly you wherever you want to go.
And part 91 is someone owning his own jet, which could include employing a pilot to fly it, but you do not sell flights to other, non-owner customers.
Did I get that correct?
So as I read the regs, you could become a PIC in part 91 and part 135 ops, if you have the right to work in the US and and FAA-ATP with necessary typerating, but no flight experience under US regulations apart from what might be necessary to hold the FAA-ATP. Correct? And once you accumulate the 1000h as SIC under 121, or as PIC under 91 or 135, you could be a PIC under 121.