I'd suggest you read the Build Tool User Guide - particularly Chapter 3 - Using the Build Tool Server. It's page 17 of the GA release of the PDFs for 8.5.
You can either auto-start it, or start it manually.
For UNIX and Linux platforms, the server mode can be started at boot time by adding the S99bldtool script to the /etc/rc3.d directory on UNIX and/etc/rc.d directory on Linux or adding this script to the Cron table. The Build Tool server opens a listening socket using the default port 7776. This listener port can be changed by adding a -p nnnn variable, where nnnn is a port number. If the listener port is changed, it must be changed in both the client and the server.
It is also possible to manually start the Build Tool in server mode, passing an unused port to communicate to the individual server. This provides the ability to run a server for an individual user, separate from the system server. It is flexible and you can start the Build Tool multiple times, each using a unique port listener address
You'll need to pay particular attention to the Security Considerations section - by default, there is no security, which is not likely to be what you want.
Another, undocumented issue is one of connections -its certainly true for Windows, and I'd expect the same for Unix/Linux. To successfully use a Remote Build Tool Server, the server has to be able to reach back and establish a connection to the client. Initial logins work fine, as does navigation, but the point where you issue the Build command requires the BT server to open a network connection to the client. If your server can't reach the client, it won't initiate the build. So this means if you're NATed (eg on a VM not et to Bridged), you may not have much luck. This was raised as a DAR, though I don't know if it survived the switch to the Ideation process.