Company builds trips based on their business reasons, anything from turns (out-and-back) from the domicile, one day (includes turns but could also have more than two legs), two, three, four, five or even six day trips with layovers. Most common trips are two and three days, turns and four days less common, five or six day trips rare (for domestic ops).

Once all of those trips are created, pilots have to bid for them.

Line bidding: The company (or the union in some cases) packages trips into pre-built monthly schedules which include enough trips to get the needed credit for each pilot so all flying is covered. You then choose (bid for) the pre-built lines in seniority order.

These lines can have a rhythm, ie same trip Tue-Fri of every week, or just be random stuff shotgunned all over the month.

PBS: All the trips go into a pool, and pilots pick individual trips which they want, and enough of them to get the specified credit range for that month. If you're very senior, you can just pick the trips you want. Most pilots use the PBS software to filter and prioritize their days off, types of trips, layovers, etc and let the software pick trips from the pool... you're limited to the trips remaining after everybody senior to you has been processed. PBS is better for most pilots, the biggy is that it allows you to bid multiple specific days off for personal/family events and the system just fills in trips around your schedule. Obviously not everybody can bid Christmas off, but you should be fine for that Dr's appointment on a Tue in Oct.