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

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

FAP (Flipper App Package) — Creating apps that can be dynamically loaded from the SD card
FAM (Flipper App Manifests) — How apps announce themselves to the system
App Examples — Various app examples, complete with the source code
Publishing to the Apps Catalog — Learn how to publish and update your app in the Apps Catalog