How do young pilots in the USA accumulate 1500-hours to become a first officer?
1 word: Instruction.

I learned to fly in the US, and all but one of the 10-15 instructors at the school were young guys who had a Commercial license and were just building hours while doing their exams and applying to airlines.

This meant they were actually getting paid (not particularly well, I hasten to add!) to build hours, rather than spending money on an airplane, or rental.

They say you don't become a pilot for the money, you do it because you love it. Nothing could be closer to the truth IMO.