This guide explains exactly what is included in the direct download on this website and how to place the JAR into a compatible Minecraft 1.21.1 NeoForge instance.
What the download contains
The packaged file is ImmersiveEngineering-1.21.1-12.4.2-194.jar. Its embedded mod metadata identifies Immersive Engineering version 12.4.2-194 for Minecraft 1.21.1 and requires NeoForge 21.1.164 or newer within the compatible release line.
Direct download vs. installer
Immersive Engineering is distributed as a JAR mod file. It is not a standalone game installer. The JAR belongs in the mods folder of the Minecraft instance that already has the correct mod loader.
Before you install
Close Minecraft, back up important worlds, verify that the target profile is Minecraft 1.21.1, and remove older duplicate Immersive Engineering JARs from that same mods folder.
After the file is added
Launch the intended profile and confirm Immersive Engineering appears in the loaded mod list before opening a valuable world. If startup fails, read the first loader or dependency error instead of repeatedly changing unrelated settings.
Next step
Once your setup is stable, use the other guides on this site to plan power, wiring, performance and larger factory systems. For the file itself, use the main direct download section on the homepage.