Go to jadenacoder/Eaglercraft-1.12.2 and download the Eaglercraft_1.12_Offline_en_US.html file (usually within a zip release).
If you're looking for custom mods or plugins for Eaglercraft 1.12, you may be able to find them on GitHub or other online platforms. Some popular GitHub repositories for Eaglercraft include: eaglercraft 112 github link
A simplified data flow: