FSSE is a save editor for Fallout Shelter. It is available for Android and Windows, however both editors can edit saves from all FalloutShelter platforms.
Downloads and more info can be found here.
This repository holds the FSSE webpage, game item data files and modding information.
FSSE consists of two parts, the save editor and the mods for the game. Using the mods is not necessary, however it can unlock new things (for example allows pet bonus editing).
The modding system is currently being rewritten from zero.