When you configure your X terminal to use XDMCP direct (query mode), you tell your X terminal the host name of the login server host. When the X terminal is booted, it automatically contacts the login server, and the login server displays a login screen on the X terminal. See your X terminal documentation for information describing how to configure your X terminal for XDMCP direct mode.
Most X servers also support the -query option. In this mode, your X server behaves as if it were an X terminal, contacting the login server host directly and requesting that it display a login screen on the X server. For example, starting the X server on a bitmap display on workstation bridget will have login server anita display a login screen on the X server:
X -query anita