system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do air marshals really spend most of their time flying around in random planes? How are they assigned? Or is that just something for the movies?
gpt
The Federal Air Marshal Service is part of the Department of Homeland Security. They are trained law enforcement officers who train specifically around planes/aviation. They go through the Federal Law Enforcement Training Center (FLETC), which all federal LEOs go through, and a secondary training specific to their role. They are assigned to specific flights, both domestic and international. Long international flights can have multiple Marshals on board. The flight crew knows of the Marshal's presence on the plane. The agency chooses which flights to have Marshals on board. They tend to increase if a large sporting event is taking place or a major political event is occurring in a city. If you fly frequently, you probably have been on a flight with a Marshal and never knew it.

Edit to answer a few more items:
Much of their time is spent in the air or at airports, but they also can have desk work.
They carry SIG p229 chambered in .357 Sig. For those not familiar, it is a hybrid between a 9mm (the bullet) and .40 (the case). Think 9mm magnum for the easiest description.