Most users will access Nexus through their standard Ansys unified installer. Nexus is included in both the EnSight and Fluent installations. Under the Ansys installation directory v221, you will find the Nexus tool launchers and scripts in v221/CEI/bin. For example, in a typical installation, the Nexus Launcher is C:\Program Files\ANSYS Inc\v221\CEI\bin\nexus_launcher.bat on Windows and is /ansys_inc/v221/CEI/bin/nexus_launcher on Linux.
Using the Nexus tools, including launching and maintaining Nexus servers, requires no additional licensing from Ansys. In years past, running a remote Nexus server required separate licensing. That has not been the case since at least 2019.