You've downloaded and installed forge right? I know that when you run multiple mods, you'll wanna have forge installed, it's basically the mod loader. Some mods will conflict with each other though, but Immersive Engineering tends to work with a lot of mods in my experiences.
I think there's another mod loader like fabric, but forge is my go-to on running mods
As for forge, you'll want forge meant for your version of Minecraft, I run on 1.12.2 myself so I use a 1.12.2 version of forge
Edit: As for .json files, the ones I work with, think of them as coding and scripting for a mod, the ones I work with determines what textures my MCA villagers will be able to use by default, and another allows me to add in custom villagers with their own trade tables.