Single list - on the day you join you become the airline's most junior pilot. If there are two (or 20) on the same intake then the numbers are allocated by age or alphabet. The order in which you are originally placed remains the same as your career progresses, so irrespective of the order in which pilots are promoted (or the time gap between promotions) the later joiner remains less 'senior' for whatever purpose 'seniority' is used.
Separate Captain & Co lists - initial bits of the above apply until you get promoted, at which stage you become the airline's most junior captain, and commence your progression up that list. Any subsequent captains are placed below you on that list - even if they joined as co-pilots before you.
LIFO - see MYT thread.