You will likely get assigned a seat during check-in (though you should still skip it if the airline tries to tack on a fee) or, at the very least, at the gate. Again: You don't have to select a seat in order to get a seat. You just might get stuck with a suboptimal one if you choose to skip it.

Seat assignment methods vary by airline and ticket type, but generally follow a tiered system. When you book a flight, many "full-service" carriers allow you to select a seat for free or for a fee based on the fare class. If you choose not to pay, the airline's reservation system (CRS) typically assigns seats automatically during the 24-hour check-in window. These algorithms prioritize keeping groups on the same reservation together, while "elite" loyalty members are often automatically moved to preferred rows. For ultra-low-cost carriers like RyanAir or Frontier, seats are often assigned randomly at the last minute to those who didn't pay, sometimes intentionally splitting groups to encourage future purchases. Southwest Airlines is the major exception, using an "open seating" model where your boarding position (A, B, or C) determines when you enter the plane to choose any available seat.