Source code is here: [https://github.com/ThomasToka/Marlin/tree/MARLIN-E3S1PROFORK-BYTT]
The most recent code is in one of the release branches.
The official pull request is here: https://github.com/MarlinFirmware/Marlin/pull/25948
This firmware is for the Creality Ender 3 S1 Pro and Ender 3 S1 Plus.
It has its own screen firmware for DWIN and Dacai displays.
I hope you like it.
Thomas Toka Developer
Please read the installation.txt in the release for full update instructions!