This is [MARLIN-E3S1PROFORK-BYTT]. A Fork of Marlin 2.1.x-bugfix with the Creality Ender 3 S1 Pro source code included. Personal tweaks, fixes and enhancements included. This firmware is for the Ender 3 S1 Pro (touchscreen) and Ender 3 S1 Plus (touchscreen).
Why do you think that https://github.com/synman/Ender-3-S1-Pro-Firmware is a good alternative to MarlinFirmware