Im working on this 20M LOC "project". Currently its multirepo and maven. They publish each dependency into a registry and depend on development snapshots, which as expected is a mess. I was thinking of turning this into monorepo and one big gradle build. But 20M locs.. IDEA craps out on me on my machine. In prior project we used to have logic in settings.gradle to create per-app local experiences. It worked but is rather janky. Doea anyone have idea how to best model this? How would big java shops like netflix etc do this?

Maybe have it as composite builds? So each of the libs a build and then the apps can include the builds they need. Then you can just open one app build in the IDE an get all included builds opened along but not more.

you then mean to have multiple settings.gradle?

Yes, each standalone build has one settings script, each project should only be directly part of one build ever, and these builds you then compose.