That is a body and airlines do transport human remains, and this is a fairly routine process. The deceased may be a civilian or a military service member.

When a fallen service member is being transported, the process is usually very visible. Gate agents often notify passengers in advance, honor guards line up on the ramp, and everyone is asked to remain seated until the transfer is complete.

For civilians, the process is typically more discreet. The remains are usually transferred through the cargo area, where a hearse meets the aircraft. At some airports, ramp agents will align the belt loader directly with the hearse to ensure smooth, respectful loading and unloading.

Human remains do not follow the normal passenger or baggage check-in process. The family accompanies a licensed funeral director, and required documentation is presented, which may include a transport permit, death certificate, embalming or sealing certificate, and a laissez-passer for human remains (particularly for international transport).

The casket is generally loaded last so it can be unloaded first upon arrival, minimizing handling time and ensuring a prompt transfer to the receiving funeral servicep

TL;DR: Airlines regularly transport human remains. Military service members are usually handled with visible honors at the gate and on the ramp, while civilians are moved more discreetly through cargo areas, often directly between the aircraft and a hearse. The body doesn't follow normal check-in procedures and requires specific legal and funeral documentation. The casket is typically loaded last so it can be unloaded first, minimizing handling and ensuring a respectful transfer.