BTW, there is no way two captains can “have the same seniority”…
That seniority number is based on when hired and that order of precedence never changes although your actual # can go up as people retire/quit/get sick etc.
This seniority allows you to bid on a “category” like 737 Captain in NYC, or A350 FO in ATL. That process happens periodically based on how your company does it as it probably requires training.
Each month you bid for schedules along with every other pilot in your category for that month. These bids are filled in seniority order allowing for various FAA/contract rest rules being followed.
As others pointed out, 737 Captain ATL is a different category than 737 FO ATL for example. Sometimes/quite often the FO may even be “senior” in the company to the Captain because that FO has chosen to stay in that category to be relatively senior and the Captain has chosen to bid that category.
Each category has trends based on the type/desirability of flying and pay rates on how senior the pilots are that want it. Generally, smaller planes with shorter leg flights go junior. Bases in the NE US go junior. After those generalities it’s really all bets off depending on what pilots want.