The server product can be downloaded via your Electronic Software Download page (or from the Claris Customer Console).
Important
Server 19.2.1 on Windows: PHP is no longer installed with FileMaker Server. After an upgrade, the folder \FileMaker Server\Web Publishing\publishing-engine\php\ is no longer present. To preserve your existing PHP installation, back up the PHP folder and restore it after the upgrade is complete.
*there could be other elements to pay attention to, I simply chose to outline this one. Make sure to read the release notes before installing the software.
More on the PHP, this time for MacOS, from the FMS 19.2.1 notes:
FileMaker Server supports HTTP/2, which provides faster web performance for web publishing including Claris FileMaker WebDirect®, FileMaker Data API, and XML. HTTP/2 is enabled by default on all versions of Windows and Linux. HTTP/2 is available on macOS Catalina or higher but must be enabled with the Enable_HTTP2.sh script, which is located in /FileMaker Server/HTTPServer/.
Note: HTTP/2 is not supported on earlier versions of macOS.
If you’re using HTTP/2 on the Mac platform, be aware of the following:
Apache HTTP Server (httpd) processes may fail when using the FileMaker Data API.
PHP web publishing is not supported.
Use the default installation directory since custom locations are not supported.
When FileMaker clients cancel finds in hosted files, finds are now halted quickly. Previously, canceled finds continued to run on the host and failed to stop promptly on clients.
Hallelujah! I can't tell you the number of times I've kicked off a find, realized it was using unstored calcs and going to take hours to run, Canceled it, had nothing happen, and eventually gave up an force-quit FMPro, losing my flow completely (and silently praying I wasn't corrupting the database in the process).