For older FileMaker Pro versions, there used to be downloadable updates on the FileMaker website, but starting with FileMaker Pro 15, they decided to have FileMaker updates on macOS go through FileMaker itself:
As you can see here, they expect your user to have admin privileges. If you're managing Macs (through Munki, for example), you may not give your users admin privileges or you may (even if your users have admin privileges) not want to bother with them running the updater themselves.
First, find a Mac that you're not doing a silent install of FileMaker Pro on. Do a regular manual install on that one Mac and make sure you have AI_DISABLEUPDATENOTIFY=0 in your Assisted Install.txt file.
Under Help, select Check for Updates....
That should show you the location of the downloaded installer.