Flipper Zero features full support for custom applications which (usually) do not require any changes to the firmware.

For easy application development, a software tool called uFBT is available.

Visual Studio Code workspace for Flipper Zero - Flipper Zero integration for VS Code
FAP (Flipper Application Package) - Creating apps that can be dynamically loaded from the SD card
Flipper Application Manifests (.fam) - How applications announce themselves to the system
Application Examples - Various application examples, complete with the source code