Hello Alberto,
The code that runs on clicking the Fiori tile is not really ABAP code. The tile opens a SAPUI5 application that is written in JavaScript. The Fiori tile has either the application URL or reference to a LPD_CUST launchpad which contains the target URL of the SAPUI5 app. This SAPUI5 app exists on the gateway server as a BSP app. Semantic objects are specifically used in Fiori for triggering navigation between different Fiori apps/navigation from launchpad to Fiori apps.
For e.g., for the Approve Purchase Orders standard delivered Fiori app there is a BSP with the name MM_PO_APV which contains the application logic of the application UI.
The SAPUI5 app interacts with the back end data using OData services - which are implemented as DPC and MPC classes in the backend system and registered on the gateway system.
You can find details of deploying a custom SAPUI5 into the ABAP repository on front end system and integrating into the Fiori Launchpad (via a Tile object) here How to Deploy UI5 Application on the Fiori Launchpad.
Regards,
Saurabh