Astarte.Device.get_pid

You're seeing just the function get_pid, go back to Astarte.Device module for more information.
Link to this function

get_pid(realm, device_id)

Specs

get_pid(
  realm :: String.t(),
  device_id :: Astarte.Core.Device.encoded_device_id()
) :: pid() | nil

Returns the pid of the Astarte.Device process for the given realm/device_id pair, or nil if there's no existing device for that pair.

Devices are registered to Astarte.Device.Registry with key {realm, device_id} when they are started.