FireMonkey for Linux, also known as FMXLinux, is an add-on component provided to Delphi and RAD Studio Enterprise and Architect edition customers via an exclusive OEM agreement with FMXLinux. FMXLinux provides capabilities for building GUI applications for Linux, extending RAD Studio’s (Delphi Edition) FireMonkey cross-platform framework.
Starting with 10.3.1, the FireMonkey for Linux (FMX) client add-on is available for Delphi and RAD Studio Enterprise customers that are current on Update Subscription via the RAD Studio GetIt Package Manager in the IDE.
Official article from Embarcadero Community
There are a number of videos that you can view for help with getting started building FMX Linux applications: