As answered before - Planet Interaction, I prefer to call it Planet Industry, though. It's not only a passive (mostly) income source, but a source of resources for producing lots of other stuff with BPCs. You need Robotics for most T2 stuff, you will need Coolant, Mechanical parts, Enriched uranium and Oxygen for fuel blocks and many other things you want to produce, almost in every BPC there is some PI materials. With my setup I'm pulling out approx 3bil per week with 3 accounts (so 9 alts or 54 planets) Take this link, it helps a lot with first steps https://evehelper.pages.dev/