My dad is a commercial pilot for a different carrier. The tl;dr is there are typically a subset of pilots on-call at any given time. Pilots who aren't on-call won't be asked to give up their weekends to cover flights.
Pilots either "sit reserve" or "hold a line" based on seniority/airframe and to a lesser degree rank (ie a pilot may choose to sit reserve as a captain when they could be a line holder as an FO). Line holders have a set schedule that they bid on (again based on seniority). If someone calls in sick, misses a flight, or if a flight goes unscheduled, scheduling contacts a pilot that is sitting reserve who then fills in the missing position.
When sitting reserve they get paid to hang out near the airport. Pilots are typically based out of some airport, reserve pilots need to be able to get to their airport within a set amount of time after being called (iirc 2 hours).
It's not that there aren't real reserve pilots, it's that there are exactly the number of backups that there needs to be under normal circumstances. Airlines don't like paying people to sit around and they've got scheduling down to a science. I think I was in high school the last time my dad sat reserve. It felt like he had to fly almost every time (but not every time!) he was on reserve.