Seniority doesn’t exist per se - there isn’t “a big list” and there is no way of finding specifically where you sit within a base or company.
There is obviously however ‘Date Of Joining’, which is used for certain admin purposes. I’m certain that the company wouldn’t order all crew into one list sorted by DOJ, but instead it would be each base sorted by DOJ, and then cuts made roughly from the bottom of each base list where crew numbers need adjusting in each location for the future.