You get better layout calculations, better performance, the new FMDeveloperTool command line tool and HTTPS tunneling for easier cooperate deployment. FileMaker is compatible to macOS Sonoma with some known issues and you may enjoy a few bug fixes. But please read details in the release notes:
FileMaker Pro 20.3.1 release notes
FileMaker Go 20.3.1 release notes
FileMaker Server 20.3.1 release notes
You can download it on the ESD website. The link is in the license email from Claris.
For MBS Plugin, just make sure you have a 13.x version. We fixed a few minor things over the beta test period, so recommended version is 13.5.