Splashtop Streamer is a key component of Splashtop On-Prem. It is required to be installed in the computer that will be remotely controlled by a client device.
For information on Streamer installation, find in:
Streamer looks like the following on Windows, it's similar on Mac.
After Streamer is installed and launched, the Streamer usually will be hidden in system tray, it can usually be found in the taskbar with the icon , It can however be hidden by the administrator in policy to avoid the user to manipulate the settings.
Splashtop Streamer is running as service, so, it has admin privilege, user can remote into the computer even in login page, and has admin privilege to handle UAC prompt.