There are two downloads for the server. An exe file for Windows and a war file that can be installed on most other operating systems. It can also be installed as a docker container.
For instructions on how to install the different versions see How to use
Windows Installer: Download 1.2
If you don't know what version to get it is probably this one.
War File (all platforms): Download 1.2
This is for use with tomcat and should work for most platforms
Docker container: DockerHub page
This is a popular third party docker container maintained by LinuxServer.io
Build from source (Free): Link
This option is totally free but you will need to re-build it if you want to update it. This one is recommended if you want to make changes to the app or don't have the play store on your phone.
Google Play ($3): Link
This one is recommended if you just want to use the app and never have to worry about updating it since it comes with automatic updates