I’ve poked around a bit and it seems like a registry is just a directory in .julia/registries with a Registry.toml. Removing/renaming that file makes the package manager no longer recognise the base registry. You can take a look at its repo to get a feel for how Registry.toml works. The gist of it seems to be a key value store of uuid = { name = <name>, path = <relative to Registry.toml>} with some more info about the package versions and its dependencies in those paths. It’s heavily interconnected with METADATA.jl at the moment, so it might be best to wait until we’ve transitioned to the new Package tracking mechanism.