It's a pack with one goal (build rocket, goto moon), that puts you in the SANDBOX that is the core of minecraft and says, "here are the tools, here's a bit of guidance on what you should automate to progress, but figure it out from tools at hand and have fun in your own way".

The mods are from different authors, but all the recipes have been tweaked and tuned to integrate with each other and provide a cohesive progression curve. Even the textures have been tweaked to make the pack visually cohesive.

A lot of recent packs give you a quest book to guide your progress and push you into the different mods slowly. You get maybe 70-80% through one mod's tech tree and then you're pushed to start on another section before you can progress into that last 20%. The Project Ozone series does a REALLY good job of making sure the different mods balance each other and interact in a way that feels good.

> you end up with a billion items

Yeah that gets bad. Most packs have some sort of inventory organization system (basically a computer you can stick all your items into with no organization and then search them up later) to help alleviate that.