Merit and seniority. You must have both.
About 2 years from joining to promotion. I think 3/4 of deliveries are done now , so it seems to have slowed.
as they promote both those without command time and those with, the process is quite involved and includes a training package and interviews/ assessments prior to the sim.
airbus doesn’t seem a hindrance, quite a few A380 captains from emirates have come .